In this report, we aim at determining the probability distribution of components demand in Alpha plants. Alpha is one of the world's top ten auto parts makers (the company's name has been altered for confidentiality issues). Based on the case of Alpha, we develop a general method of components requisitions and forecasts analysis in Assemble-to-Order systems. This method allows to find the probability distribution of components demand and to estimate the parameters of this distribution. We apply the proposed method to the case of an Alpha plant situated in the west of France. We find that component demand follows a compound Poisson distribution.