IRT [item response theory] models require validity conditions which turn out to limit their applications. The IRT technicity, the stakes they are aimed at have not always given the opportunity of argued debates about their limits. However, as far as orientation or certification procedures are concerned, the precision level of evaluations effectuated on the basis of IRT models is very important since, in those aspects, students results can have heavy consequences for their future. That is the reason why this study will try to determine which are the features required for a sample of students or of items in order to secure the quality level of Birnbaum’s model estimates.