İn thls paper, we mainly concentrated on spocial types of bela distributlons which are employed İn probabillstlc PERT models. Contrary to tho exletir>g literatüre, it is claimed that the mostllkely or modal value of the beta distributlons should not be estimated. Because. for a given set of parameters and an estimated range. the modal value of the beta distribution is completely determined. Thus, dependlng on the magnltude of the dlfference between the estimated modal value and the cal-culated one, expected completlon time of each actlvity and that of the project may dlffer greatly from the one that coüld be obtained othervvise. Thls dlfference conseçuently, may cause selectlon of an Incorrect crltical path and misleading comple-tion time probabllities.