1 For example, the disorder sickle cell anaemia is caused by a mutation in the gene that instructs the building of a protein called haemoglobin. 2  This causes the red blood cells to become an abnormal, rigid, sickle shape. 3  However, in African populations, having this mutation also protects against malaria.
Optimistic time - generally the shortest time in which the activity can be completed. It is common practice to specify optimistic times to be three standard deviations from the mean so that there is approximately a 1% chance that the activity will be completed within the optimistic time.
Calculating Slack Time. Calculating the slack time for an activity in a PERT chart is very easy and does not involve any complex calculations. There are two things that are needed to calculate the slack time: 1  ES â the earliest time when an activity can be started.  LS â the latest time when an activity must be started.