Photometric data on flares of cluster flare stars published by different observers were - as a rule - collected during decades by different instruments and amidst various conditions. The uncritical use of these data in global statistical studies is usually unfair since such a collection is an indefinite mixture of high and low quality results. In order to make these investigations more reliable even the best and most complete photometric catalogues have to be filtered.

The first step of the procedure described in the paper is rejecting all flare events having too small amplitudes - seeing that they are under-represented in the catalogue as opposed to medium and large amplitude flares. Then the photographic coverage of the aggregate has to be checked and all events observed in partially covered regions have to be excluded. Next any non-randomness in the time distribution of flares of each active object is to be unveiled and flares of the doubtful stars have to be omitted. Finally some objects are to be rejected for their individual characteristics.

The usefulness of the method is demonstrated with the example of the Pleiades, one of the richest open clusters known. More than 3000 hours of effective observing time has been devoted to flare photometry in fields centered upon Eta Tauri and the outcome of the programme the CPFS - the most complete catalogue of flare stars of the region - contains data of 519 flare stars and their 1532 flare events. Earlier versions of this catalogue or subsets of its data have been used to estimate the probable number of flare stars in the Pleiades region. Results of these attempts range from 600 to 2700 partly owing to the unreliability and incompatibility of the data sets. The method presented here is able to give much more reliable estimates but of course in the limited largest common field (LCF) only. As a result of the filtering the LCF contains 77% of the objects and 67% of the flares listed in the CPFS. Time sequence analysis of the 1026 flare events observed on 402 flare stars and statistical modelling of the phenomena suggested in an earlier paper fit each other almost perfectly.

The conclusion of the study is that the most likely number of flare stars in the reach of the telescopes used in this programme is between 520 and 600 in the LCF of the Eta Tauri fields. Their average flare frequency is about 1/2400h and the CPFS lists 70% of these objects. In order to be able to compare the Pleiades field flare stars and those of other aggregates the method of filtering will be carried out throughout the analysis of the most extended data sets available.