The FIRBACK survey is a deep imaging survey carried out with ISOPHOT on board ISO, at 170 μm. The survey covers ≈4 sq deg in three high galactic latitude fields (FIRBACK South Marano, FIRBACK North 1 and FIRBACK/ELAIS North 2). In these fields, fluctuations due to unresolved extra-galactic sources were isolated for the first time [1]. We are using a P(D) (probability distribution of fluctuations) analysis to gain information about the N(≥S) curve of the unresolved sources which make up the background. Many planned surveys (e.g. SIRTF, FIRST, PLANCK...) will pose the same analysis challenges and we hope that the outcome of this study will be useful in that context. As a preparation to the analysis of the FIRBACK maps we analyze simulated sky maps. We create histograms of number of pixels per flux bin. The histograms are binned such that there are enough pixels per bin to allow us to apply Gaussian estimates for the errors. We then calculate the expected number of pixels within the relevant flux ranges and evaluate the χ2 statistic. We will use a maximum likelihood algorithm to find the parameters of N(≥S).