3.1. Baseline gender classifier

Convolutional neural networks (CNN) have performed considerably better in terms of training time than other networks by peaking a better validation accuracy for small datasets with more consistency. We considered a 3-channel CNN layer architecture with long-short-term memory (LSTM) layers on top for our specified gender classification task. The purpose of using a multi-channel CNN architecture is to proceed with sentences in different resolutions (or n-grams) at each time step by defining different kernel sizes for each channel’s convolutional layer. Although CNNs are generally used in computer vision, they have performed exceptionally well in capturing input text patterns. The necessity of LSTM layers’ presence in our architecture is that the model needs to memorize these extracted patterns. Therefore, by locating LSTM layers on top of convolutional layers, LSTM fulfils such demand (model visualized in Figure 2). Since our experiments demonstrated significantly minor improvements in Persian gender style transfer when accounting for punctuation and stop words, we remove them in the preprocessing phase before training the aforementioned 3-channel model on gender-labelled text. Referring to a sentence’s source and target styles as $S_s$ and $S_t$ , we transfer $S_s$ to $S_t$ in Sections 3.2–3.5.

Figure 2. Proposed baseline gender classifier’s neural network architecture.

3.2. Detecting gender style representative tokens

Many previous studies have underscored the role of specific part-of-speech tags as vital indicators in the determination of an author’s gender (Ishikawa Reference Ishikawa2015; Bozic Lenard Reference Bozic Lenard2016; Slavova, Atanasov, and Andonov Reference Slavova, Atanasov and Andonov2016; Garimella et al. Reference Garimella, Banea, Hovy and Mihalcea2019). In many languages, notably in our Persian corpus, each gender-tagged sentence contains a specific set of words that play the most prominent stylistic roles. These words are typically classified as either adjectives or adverbs. The distinguishing factor between the two genders largely lies in the author’s choice of words within these particular part-of-speech tags. Adjectives, recognized as the most decisive part-of-speech tag, consist of different types that vary based on the language in question. Here, we provide a list of the types of adjectives as found in English and Persian languages.

• English: Adjectives are grouped either as Descriptive (e.g., I have a fast metabolism) or Limiting (e.g., I saw four cars). Although the former describes the quality of the noun, the latter limits it. Descriptive adjectives bifurcate in terms of where they are located. If the adjective appears directly beside the noun it describes, it is called an attributive adjective (e.g., The restaurant has a remarkable view!) and if connected to the noun with a linking verb, they are called predicate adjectives (e.g., The pizza was too salty!). Besides definite & indefinite articles, limiting adjectives are categorized as 8 different subgroups of possessive, demonstrative, indefinite, interrogative, cardinal, ordinal, proper, and nouns used as adjectives.

• Persian: Similar to English and most other languages, Persian adjectives also consist of descriptive adjectives. The only exception is that unlike English, attributive adjectives come after the noun. Other Persian adjective types are cardinal, ordinal, interrogative, indefinite and exclamatory adjectives.

In addition to adjectives, we also include verbs and nouns, since, as shown by Garimella et al. (Reference Garimella, Banea, Hovy and Mihalcea2019) and Slavova et al. (Reference Slavova, Atanasov and Andonov2016), gender differences can also exist between these parts of speech. Thus, by identifying adjectives, adverbs, verbs and nouns present in an input text, we create a set of gender representative tokens that we will replace with those of the opposite gender and make progress towards the goal of gender style transfer.

To detect the specified tags, we used a part-of-speech tagger that would tag each token of a sentence to search for an elegant replacement token from its opposite gender in the subsequent step. We used Parsivar (Mohtaj et al. Reference Mohtaj, Roshanfekr, Zafarian and Asghari2018) and Spacy (Honnibal and Johnson Reference Honnibal and Johnson2015) language processing toolkits’ part-of-speech taggers to do so on our Persian and English corpora, respectively. The Parsivar toolkit is reported to be 95% accurate.

3.3. Extracting similar tokens as replacement candidates

By detecting gender style representatives of an input text, we look after replacements from which we may bear down on style transfer purposes. We used fastText word vectors as a pre-trained word embedding that was trained using continuous bag of words with character n-grams of length 5, a window of size 5 and 300 in dimension, specified to return the $\text{top}_n$ most similar words of a given token.

(1) \begin{equation} \text{sim} = \cos\!(\theta )={{\bf A} \cdot{\bf B} \over \|{\bf A}\| \|{\bf B}\|} = \frac{ \sum _{i=1}^{n}{{\bf A}_i{\bf B}_i} }{ \sqrt{\sum _{i=1}^{n}{{\bf A}_i^2}} \sqrt{\sum _{i=1}^{n}{{\bf B}_i^2}} } \end{equation}

Given A and B as two predetermined words’ word vectors and $\textbf{A}_i$ and $\textbf{B}_i$ as their vector components, the cosine of the angle between word vectors is calculated using Equation (1). By considering $A$ as a gender style representative’s word vector and $B$ as for all other available word vectors in the fastText’s vocabulary, we apply this computation on all $(A, B)$ pairs. Altogether, whenever a token is categorized as either adjective or adverb (or any other specified tag), we pass the token to the $most\_similar$ built-in function of fastText to obtain a set of suggested replacement tokens with their specific similarity rates to choose between. However, to transfer a sentence’s style, we have to select replacement tokens from the opposite gender suggested ones. This is where our character-based token classifier indicates the gender from which the suggested tokens are derived.

3.4. Selecting target styled candidates

Besides classifying sample sentences, we need to train a new model to classify tokens as either male or female. This opportunity allows us to waive those suggested replacement tokens with the same style as $S_s$ and leave the set only with the $S_t$ styled ones. We used a sequential neural network with a convolutional layer and a LSTM layer on top. Each input token is represented using character-based representation, where each character is encoded using a one-hot vector. A visualization of this model is shown in Figure 3.

Figure 3. Proposed character-based token classifier’s neural network architecture.

The reason behind using a character-based model when classifying a single token is to handle the unfortunate probability that a token is out of embedding’s vocabulary (OOV) or is misspelt (Nguyen, Ngo, and Chen Reference Nguyen, Ngo and Chen2020). In either case, as long as the model is character-based, it will automatically determine the best pattern to digest the token and represent it as a vector. In addition, character-based models are better at capturing underlying emotions from an input than word-based models (Zhang, Zhao, and LeCun Reference Zhang, Zhao and LeCun2015). As Jandl et al. (Reference Jandl, Knaller and Schönfellner2017) suggest, characters can convey emotions such as passion, envy and nervousness. Hence, processing documents at the character level can potentially lead the system towards obtaining a deeper understanding of input text. Furthermore, word-based models for token classification are much more likely to become biased towards a specific output label.

Findings on gender differences in Persian have revealed how male and female writers adhere to non-identical linguistic forms, resulting in the selection of words with varying frequencies (Rasekh and Saeb Reference Rasekh and Saeb2015; Jouya, Sayadian, and Naeimi Reference Jouya, Sayadian and Naeimi2018). This phenomenon in part is due to the nature of the Persian word, which, as opposed to the English, focuses much more directly on the gender of the subject than is seen in English. For example, the term “mother-in-law” refers to the mother of one’s spouse and can be used by both genders. While in Persian, there exist two versions of this term where one addresses the mother of one’s husband () or the mother of one’s wife (). The gender-differentiated usage of Persian words thus opens up the possibility of learning their hidden gender patterns based on usage frequency. Hence, we label words according to the gender of the authors that used them more frequently. Next, we feed the aforementioned character-based model with all extracted adjectives, adverbs, verbs and nouns from both male/female sentences. As a result, the model would categorize the word embedding’s suggested tokens as either male or female. A visualized result of applying this model on a word embedding space is shown in Figure 4.

Figure 4. An example of fastText word embedding space that has been projected with PCA. Note: each blue/red scatter represents male/female classified. (Note: Persian pronunciations are shown between slashes, and English translations are included between parenthesis.)

It is worth to mention that extra filters are applied on specific part-of-speech tags’ suggested lists. For instance, we step through each replacement candidate of a verb and remove those with the same stem or lemma. This allows us to only focus on candidates with different origins. Additionally, we transform each candidate to comply with both tense and form of the original token for which it is suggested as replacement. Hence, the method avoids replacing tokens with candidates that will damage the flow and fluency of the original text.

3.5. Returning a fluent combination

As of now, we have a set of target-styled tokens for each word, and our only concern is to choose the most fluent combination. The search for such a fluent sentence heavily requires an optimized algorithm and heuristic. Our developed algorithm is based on Langkilde and Knight (Reference Langkilde and Knight1998) as well as the beam search algorithm, two of the algorithms heavily studied in machine translation.

To keep up the fluency of a given sample, we keep track of all unigram, bigram, trigram and 4-gram counts in our baseline gender classifier’s train set. To do so, we define a dictionary and iterate over all sentences and extract their specified n-gram counts and assign them as keys and their counts as values. This dictionary will further be used in the scorer function below:

(2) \begin{equation} \text{BeamScore} = \frac{4 \times \text{fg} + 3 \times \text{tg} + 2 \times \text{bg} + 1 \times \text{ug}}{4 \times 10 \times (1 - \text{sim})} \end{equation}

Given fg, tg, bg, ug and sim as the 4-gram, trigram, bigram, unigram and the similarity rate (calculated by word embedding for each of the suggested tokens), we calculate BeamScore (Equation 2) which is the mean of standardized n-gram counts and divides it by the dissimilarity of the tokens. We designate the efficacy of 40%, 30%, 20% and 10% to each of the 4 to 1-gram counts, respectively. However, the dilemma here is to score the first and the last words of a sentence and examine a suggested token’s score to start or end a sentence with. Therefore, the two tags of <START>and <END> are added as tokens to each sentence’s beginning and end to overcome the problem. Having defined the root as <START> and replacement tokens as nodes, the algorithm calculates at each step the beam score based on the traversed path’s last three nodes and the visiting node until it reaches the <END> node. By the end, the algorithm returns a fluent combination of tokens since we extracted the most probable sentence and transferred an input sample from $S_s$ to $S_t$ meanwhile. The pseudocode of our implemented beam search is given in Algorithm 1.

By transferring the test set and passing it again to our gender classifier model, we measure our model’s accuracy loss on $S_t$ text, which it formerly predicted in their $S_s$ style. An overview of our style transfer approach is visualized in Figure 5. We have made all implementations and models of this paper publicly available at its GitHub repository.Footnote ^a

Figure 5. An overview of what our model’s approach for transferring an input’s style from $S_s$ to $S_t$ does. (Note: for an input with five tokens, we have $t_{1..5}$ and each $t_i$ has a set of $r_{ij}$ with j as the number of opposite gender predicted set of the token classifier between the $\text{top}_n$ =10 word embedding’s most similar suggested words.)

4.1. Experimental setup

Datasets: Despite the focus of this study being Persian, we also apply our model to an English dataset to compare its performance with state-of-the-art English gender style transfer models. The following two datasets of Formal Gender-Tagged Persian Corpus by (Moradi and Bahrani Reference Moradi and Bahrani2016) and Gender (English) by (Reddy and Knight Reference Reddy and Knight2016) are used for our experiments. The Persian dataset contains 132,410 and 325,205 sentence-level samples from male and female authors, respectively. In contrast, the Gender-tagged Yelp dataset is constructed out of sentence-level reviews of different food businesses, each categorized as either male or female. A comparison between the two datasets is shown in Table 2.

Table 2. Dataset comparison

Hyperparameters: There is an embedding, a convolutional layer and an LSTM layer prepared in each of our baseline gender classifier channels ’s with the output dimension of 100 for embedding layers, 32 filters, a dropout rate of 0.5 and a maximum pool size of 2 for convolutional layers and 256 hidden units with a recurrent dropout rate of 0.2 for LSTM layers. To process texts at different N-gram levels, each channel’s convolutional layer has different kernel sizes of 4, 6 and 8. The model is trained for 10 epochs with these hyperparameter choices.

In our character-based token classifier, we sequenced the same setup in a single channel as the previous model, a convolutional and LSTM layer is stacked up, with 32 filters, 8 in kernel size, max pool size of 2 for the convolutional layer and 125 hidden units for the LSTM layer. This model was trained for the same number of epochs as our baseline model with no dropouts.

With its learning rate set to 0.001, the Adam optimizer (Kingma and Ba Reference Kingma and Ba2015) was used in both models, as it is better at handling sparse gradients.

Evaluation Metrics: Assessing methodologies for text style transfer has proven to be a complex problem (Mir et al. Reference Mir, Felbo, Obradovich and Rahwan2019). The concept of style is hard to define, and this has resulted in the use of various evaluation metrics, aspects and methods. Accordingly, the justification of the style attribute while maintaining content comes with the complexity of choosing the right evaluation setup to enable comparison. Therefore, to overcome this challenge, different evaluation metrics have been presented. Given $x$ and $x^{\prime }$ as the original and transferred text, Fu et al. (Reference Fu, Tan, Peng, Zhao and Yan2018) introduced the following metrics which correlate considerably with human judgements:

1. 1. Transfer Strength: Motivated by Shen et al. (Reference Shen, Lei, Barzilay and Jaakkola2017), they used a classifier based on Keras examples, which measures transfer accuracy.

2. 2. Content Preservation: To evaluate the similarity between $x$ and $x^{\prime }$ , they calculated the cosine similarity of their relative embeddings.

Similarly, Mir et al. (Reference Mir, Felbo, Obradovich and Rahwan2019) introduced the following:

1. 1. Style Transfer Intensity: Having mapped $x$ and $x^{\prime }$ to their style distributions, it quantifies the style difference of their distributions to alleviate evaluation.

2. 2. Content Preservation: Unlike Fu et al. (Reference Fu, Tan, Peng, Zhao and Yan2018), they utilize BLEU (Papineni et al. Reference Papineni, Roukos, Ward and Zhu2002) to measure the similarity between $x$ and $x^{\prime }$ .

3. 3. Naturalness: having passed $x^{\prime }$ to its function, it quantifies to what extent the transferred text is human-like.

Hu et al. (Reference Hu, Lee, Aggarwal and Zhang2022) used the perplexity score (PPL) and the style transfer accuracy (ACC) to measure the fluency and transfer strength of $x^{\prime }$ . Additionally, Word Overlap (WO), Cosine Similarity and self-BLEU were used to measure content preservation. Finally, they summed up all metrics in the following two:

1. 1. Geometric Mean ( $G\text{-Score}$ ): Which is equal to the mean of $1/\text{PPL}$ , WO, ACC and self-BLEU. (Note that they calculated the inverse of PPL since lower PPL is preferred and that the cosine similarity metric is not included in the mean due to its insensitivity)

2. 2. Harmonic Mean ( $H\text{-Score}$ ): The Harmonic Mean of the above sub-metrics is calculated to highlight different priorities when evaluating.

It is worth mentioning that Yamshchikov et al. (Reference Yamshchikov, Shibaev, Khlebnikov and Tikhonov2021) proved in a recent study that fastText and Word2Vec pre-trained embedding vectors should not be used to evaluate text style transfer approaches in terms of content preservation. They demonstrated how such evaluation pipelines suffer from inaccurate content prediction, in analogy to similar human judgements. However, in this study, we have used only cosine similarity as one of our proposed method’s crucial components to handle content preservation, but, in terms of evaluation, we left decisions to human annotators. Additionally, we utilize ACC, BLEU, PPL and a metric similar to naturalness in our automatic and human judgements as well. The details of our evaluation metrics are broken down in detail in Section 4.

Table 3. Model comparison

4.2. Model evaluation

To acquire the highest accuracy possible, we have stepped through different models and architectures. The process of choosing our gender classification model was based on a comparison between Naïve Bayes, Logistic Regression, Multilingual BERT, SVM, CNN and CNN + LSTM as baseline classifier. As shown in Table 3, our proposed baseline classifier architecture peaked the highest accuracy for our classification problem on Persian text with 90% and 81% in English. We finalize our model here since an efficient model in both languages is our main concern.

Due to the relatively small amount of data in our Persian corpora, a probabilistic model like Naïve Bayes performs poorly with very low precision and recall, as long as the frequency-based probability estimate becomes zero for a value with no occurrences of a class label.

Table 4. A comparison of the effects of the developed style transfer approaches in defeating the gender classifier model

Pre-trained language models like bi-directional encoder representations from transformer (Devlin et al. Reference Devlin, Chang, Lee and Toutanova2019) or BERT have had a significant rise due to their success in topping state-of-the-art natural language processing tasks. However, at the time of our research, there has not been any Persian-specific pre-trained transformer language model introduced. However, among proposed pre-trained models of BERT, multilingual cased contains the top 100 languages with the largest Wikipedias including Persian (Farsi). But, as shown, fine-tuning it did not have the same durability as training a classifier from the scratch.

A logistic regression model is a generalized linear model that could be reminded of a neural network with no hidden layers. Evidently, a neural network model with such hidden layers as convolutional and LSTM carries more advantages in solving our problem. convolutional layers perform outstandingly in pointing out tokens that are good indicators of an input’s class, and LSTM layers in associating both short and long-term memory with the model, thus resulting in a better accuracy score in an analogy to support vector machine (SVM) algorithm (Moradi and Bahrani, Reference Moradi and Bahrani2016).

When designing a token classifier, casting each token to its 300-dimensional embedding space representation as model inputs results in contextual information loss, making a word-based model an inappropriate choice for classifying tokens as male/female. On the other side, designing a model on a character level fills the gap and distributes tokens by stylish criteria in a better way.

Achieved experimental results by repassing the test set to our gender classifier depicts our research’s success the most. It demonstrates how have different approaches resulted, using different models and architectures on our set. We call our gender classifier’s $S_s$ test set as $D_a$ and divide it into two different subsets: (1) $D_r$ , which includes all samples that have been predicted correctly by the model, and (2) $D_w$ , which consists of all samples that have been mispredicted by the model. Besides, by transferring all $D_a$ samples to $S_t$ , we name the transferred set as $T_a$ , $D_r$ as $T_r$ and $D_w$ as $T_w$ . At inference, each of the three $D_a$ , $D_r$ and $D_w$ sets have their own accuracy scores when solely passed to the classifier. In Persian, the model yielded 90% in $D_a$ , 100 % in $D_r$ and 0% in $D_w$ . Additionally, in English, the model performed 90% in $D_a$ , 100 % in $D_r$ and 0% in $D_w$ (when the model is solely evaluated on $D_r$ and $D_w$ sets, it achieves 100% and 0% mainly because they are subsets of the $D_a$ containing samples that the classifier correctly and incorrectly predicts). In Table 4, each approach’s name contains two vital information: (a) what its token classifier model was based on [word or character (Septiandri Reference Septiandri2017)] (b) what tags are supposed to be identified by our part-of-speech tagger to be changed into their opposite gender. Our goal is to defeat the gender identification model by transferring a sentence’s style to its opposite gender, thus diminishing the gender identification model’s accuracy. This means that the gender classifier mistakenly predicts male authors as female and female authors as male, which demonstrates the success of our style transfer approach. As shown in the table, it has been clearly demonstrated how the primary approach has been elevated by changing its different components. The more robust our token classifier and the more varied our part-of-speech tags’ scope gets, the weaker the gender identification model’s performance gets. In a task similar to ours, Septiandri (Reference Septiandri2017) demonstrated that character embeddings perform better at classifying full names and first names according to gender. Table 4 demonstrates the same effect, with the character-level model outperforming word-level models.

Table 5. A comparison of positive and negative effects of applying different approaches

As mentioned, after transferring the $D$ sets, we obtain $T_a$ , $T_r$ and $T_w$ . The success of a style transfer approach is expected to result in a decrease in $T_r$ accuracy and remain the $T_r$ accuracy unchanged. The former shows that the transfer approach has successfully faked the gender classifier and makes it mistakenly predict samples it used to correctly classify their authors before their transfer. The latter, ideally, should remain unchanged (0%) and not be increased. However, an increase in the $T_w$ accuracy means that the transfer has failed and unintentionally aided the classifier. Accordingly, the first approach resulted in a 4% decrease overall, 8% on faking correct predictions, but unintentionally helping it correctly predict those it had mistaken before. Meaning the approach has helped the model instead of faking it. Given the number of samples that faked the model as $f$ , the number of samples that unintentionally helped the model as $h$ and the size of the test set as $n$ , we define the trade-off value in Equation (3). This formula is designed to capture the effects of both $f$ and $h$ to ease comparison between approaches (Table 5). With this formula, those which fake the model more and help it less, receive higher scores.

(3) \begin{equation} \text{Trade-off} = \frac{f-h}{n} \times 100 \end{equation}

As shown in Table 6, Persian $D_a$ contains 45,762 test samples in which 41,185 samples (90%) were correctly guessed and belong to set $D_r$ and 4577 samples (10%) guessed incorrectly by the gender identification model, which belongs to set $D_w$ . As shown in Table 5, there was an 8% decrease in $D_r$ ’s accuracy (3295 samples) and a 15% rise in $D_w$ (686 samples), resulting in a trade-off value of 5.70, which demonstrates its lack of ability in defeating gender identification. But as we go along testing approaches 2, 3 and 4, we get back the trade-off values of 7.77, 34.33 and 36 in Persian. The major leap between the second and third models’ trade-off values represents the pivotal role that a bigger scope of part-of-speech tags plays. In addition, switching the token classifier’s processing level from word to character in the fourth approach also shows how it affects efficiency. The same evaluations have been made in English where the final approach yielded the results of 34%, 61% and 37% for $T_a$ , $T_r$ and $T_w$ , and the trade-off value of 24.56. Specific contingencies of applying our finalized approach [i.e., Character-based + (Adj, Adv, V, N)] on our defined sets of $D_a$ and $T_a$ in both languages are shown in Table 5, which demonstrates the number of samples that were predicted either correctly or incorrectly by our baseline gender classifier.

In order to prevent the transfer approach from unintentionally helping the classifier to predict the samples correctly, amplifying our character-based token classifier is the most rational alternative, since converting a sample’s content to its target style is its primary essence, and content is what the classifier is obligated to detect.

Table 6. A contingency table on our finalized style transfer approach [i.e., character-based + (Adj, Adv, V, N)] in Persian and English

4.3. Statistical evaluation

To determine whether there is a significant difference between the means of the two gender labels, we use a statistical hypothesis testing tool called t-test (Kim Reference Kim2015) to assure that there is not an unknown variance and that labels are all distributed normally.

In order to assure if the two gender labels come from the same population, by taking samples from each of the two labelled sets, t-test hypothesizes that the two means are equal. By calculating certain values and comparing them with the standard values afterwards, t-test decides whether inputs are strong and not accidental or that they are weak and probably due to chance, resulting in rejection and acceptance of the hypothesis, respectively.

(4) \begin{equation} t = \frac{\overline{X}_D - \mu _0}{s_D/ \sqrt{p}} \end{equation}

We use inputs in both original and transferred style in Equation (4) to measure the t statistic value for dependent paired samples in which $\overline{X}_D$ and $s_D$ are each pair’s average and standard deviation of their difference, $p$ as the number of pairs, and $\mu _0$ as hypothesized mean, which we assign to zero when testing the average of the difference.

The p-value is the probability of obtaining an equal or more extreme result than the one obtained when the hypothesis is true. Significance level (or alpha) is a threshold value which is the eligible probability of making a wrong decision (rejecting the hypothesis). We have calculated p-values in both languages by considering n− 1 degrees of freedom and assigning 0.01 to alpha. As demonstrated in Table 7, test results are significant in both languages with their acquired p-values.

Table 7. P-values for paired samples in our corpora (alpha = 0.01, degree of freedom = n − 1)

4.4. Human evaluation

A proper assessment of the generated text is necessary to prove its correctness. We considered facets like $fluency$ and $semantic$ that each sample had to be assessed based on. The former facet determines whether a given text is reasonably close to legible human language or that it is presented in an indecipherable manner. As the name implies, the latter is employed to evaluate inputs depending on conceptual meanings when interpreted. We additionally added an $adulteration$ facet to determine whether the given text seemed adulterated or not. Most importantly, annotators were also asked to assign 1 to samples where they believed the author was male and 0 to samples where they believed it was female which is indicated as the $gender$ facet. We randomly selected 300 inputs that included 75 $S_s$ and 75 $S_t$ styled samples in both English and Persian. By shuffling inputs and dividing them by three, we assigned each 100 samples to an annotation group with three different annotators. Annotators were then asked to rank each sample based on our set of criteria in binary form. The reason for using $S_s$ texts from the English and Persian corpora in human evaluation is to draw a comparison between how ground-truth samples are overall ranked. This then makes it more comprehensive to understand human judgements among $S_t$ samples.

4.4.1. Inter-annotator agreement

Before particularizing annotations, we test inter-rater reliability with kappa (Viera and Garrett Reference Viera and Garrett2005), a standard measure of inter-annotator agreement which aims to compare the amount of agreement that we are actually getting between judges to the amount of agreement that we would get purely by chance.

By letting $N$ be the number of samples and defining $R$ and $I$ as two sets of agreed and disagreed samples for each of the three annotators in a specific group, $A$ would be the set of samples where all three annotators agreed on and $P(A)$ and $P(E)$ as fractions of real and accidental agreements.

(5) \begin{equation} A = (R_1 \cap R_2 \cap R_3) \cup (I_1 \cap I_2 \cap I_3) \end{equation}

(6) \begin{equation} P(A) = \frac{|A|}{N} \end{equation}

(7) \begin{equation} P(E) = \left(\frac{R_1}{N}\right)\left(\frac{R_2}{N}\right)\left(\frac{R_3}{N}\right) + \left(\frac{I_1}{N}\right)\left(\frac{I_2}{N}\right)\left(\frac{I_3}{N}\right) \end{equation}

(8) \begin{equation} K = \frac{P(A) - P(E)}{1 - P(E)} \end{equation}

K value is calculated for each facet in every group. Finally, their average score is stated in Table 8 which demonstrates the stability of annotations since such obtained results are counted as $substantial$ ones in Kappa inter-annotator agreement’s jargon.

Table 8. Results of Kappa inter-annotator agreement

4.4.2. Quality assessment

Since our annotator groups each consists of three different annotators, when classifying each sample’s facet, we consider the two most agreed on opinion as the sample’s final class (e.g., if at least two out of three annotators classified a sample as 1 in fluency, we call that a fluent sample). Table 9 demonstrates random samples’ quality assessment based on their language and style. The important thing to note in Table 8 is that the source samples in both languages are not identical as “ground-truth style transferred”. Meaning we do not aim to approach those accuracies. They are simply raw text from the dataset (not transferred) that have been given to annotators to be assessed alongside the transferred samples. The reason we have also asked the annotators to assess raw dataset text is to provide them with an unbiased annotation scheme towards transferred-only outputs and demonstrate the evaluation confidence of annotators.

Table 9. Quality assessment of annotated samples

Fluency: As mentioned in Algorithm 1, when choosing the right combination among all suggestions, we prioritize tokens with the highest frequency in different n-gram scopes (Equation 2) when choosing among replacements. This strategy will lead us towards a fluent $S_t$ sample which is clearly proved by annotations shown in the table, with high accuracies of 77% and 75% in Persian and English.

Semantic: Our results have marginally lower semantic accuracy compared with the samples’ overall fluency, which is probably due to the literary nature of the Persian and the informality of the English corpora. In spite of the difficulty of replacing intended tokens in such non-identically styled corpora in the two languages, our approach demonstrates its compatibility with such diverse texts.

Adulteration: The rationale for using $S_s$ samples in annotating process was to see if $S_t$ samples seemed evidently adulterated among the others, or that they obeyed of a similar format, which surprisingly, nearly the same amount of $S_t$ English samples was detected as adulterated (28.95%) as the $S_s$ English ones (26.67%), which heralds of low difference among them. Since our acquired pre-trained Persian word embedding is not as well-trained as pre-trained English word vectors, $S_t$ Persian samples appear in greater frequency (40.54%) than $S_s$ Persian s (21.33%). Additionally, the cumbersome structure of Persian literary texts makes it even more difficult to deal with during the transfer process.

Gender: Having asked the annotators to label each sample as either male (1) or female (0), we then compared their answers to their true labels to see how accurate human judgement is in determining the gender of authors. As indicated, the gender can be more accurately predicted in Persian rather than English. This is potentially due to the substantial linguistic differences between Persian and English which made our model more efficient in the former (80.53%), for which it was exclusively developed, than in the latter (68.95%).

4.5. Automatic evaluation

To indicate our proposed method’s correctness, we assess our method via automatic evaluation measurements in different aspects of fluency, content preservation and transfer strength. Previous works measured transfer strength with a style classifier, explicitly trained for evaluation purposes. But since proposing such a classifier was one of our key contributions, our model has already been particularized in previous sections. This helps us measure the loss of accuracy (the division of the number of correctly guessed authors’ genders by size of the test set) caused by the style transfer approach. The higher the loss gets, the better the approach has performed. We also employed BLEU (Papineni et al. Reference Papineni, Roukos, Ward and Zhu2002) and OpenAI GPT-2 language model, respectively, to measure content preservation and fluency of our transferred set. Although evaluating with such automatic metrics like BLEU is inadequate if being applied single-handedly (Sulem, Abend, and Rappoport Reference Sulem, Abend and Rappoport2018), it benefits us with a general understanding of how preserved a $S_t$ sample’s content is. But in fluency terms, we calculated the perplexity of those samples using GPT-2 language model.

As a comparison with previous work, we intend to compare our method (PGST) on the analogy of three other previously proposed methods. Prabhumoye et al. (Reference Prabhumoye, Tsvetkov, Salakhutdinov and Black2018) came up with the idea of adversarial mechanism and Back-Translation (BT) and Sudhakar et al. (Reference Sudhakar, Upadhyay and Maheswaran2019) proposed B-GST and G-GST which respectively were blinded and guided towards particular desired $S_t$ style attributes using transformers (Vaswani et al. Reference Vaswani, Shazeer, Parmar, Uszkoreit, Jones, Gomez, Kaiser and Polosukhin2017) which at the time of writing this paper and to our best of knowledge, is the state-of-the-art on our mutual English gender-tagged dataset.

When making an analogy, the key factor is not to consider each evaluation metric separately but to contemporaneously assess them all together. As shown in Table 10, in terms of target-style accuracy, the BT model performs admissibly well, but its generated text does not preserve much content, thus resulting in a low BLEU score, whereas in $S_t$ style matter our method almost obtained the same result as state-of-the-art method collateral, G-GST, but with much lower perplexity and higher BLEU score comparing to the prior state-of-the-art method, BT. All in all, it can be concluded that besides other monolingual methods, even with a much simpler foundation to a multilingual extent, our proposed method has achieved reasonable success in English, whereas our main focus was on devoting such a method in Persian.

Table 10. Comparison of automatic evaluation results of different models in English