What are the 3 different time estimates in PERT?

In PERT 3 different time estimates are defined for each activities.

(i)      OPTIMISTIC TIME ESTIMATE :

          This is the shortest time within which an activity can be completed.

Conditions better than normal are expected during the execution of activity and no provisions are made for delay. This time estimate is usually denoted by ‘t_o’

(ii)     PESSIMISTIC TIME ESTIMATE :

          This is the maximum time required for an accomplish on of an activity i.e. It is the time required if the activity is performed under totally abnormal or adverse condition.

It is denoted by ‘t_p’  

(iii)    MOST LIKELY TIME ESTIMATE :

          This is the estimate of the type which an activity will require under normal circumstances. It is usually denoted by ‘t_m’.

The frequency curve of the activity time estimate resemble the b distribution curve with t_o and t_p as an extreme point and tm as the nodal value. Hence the mean time require for the completion of an activity is given by.

Mean Time = to +4 tm +tp/6