2. • Single time estimates
• Didn’t allow for any variation
• AT treated as if they are known for
certain
COMPARISON CPM AND
PERTREVIEW
• Multiple time estimates
• Allow for any variation
• AT treated as probabilistic
PERT uses probabilistic activity times
CPM PERT
3. Approach to estimating activity times
WHAT IS PROBABILISTIC ACTIVITY
TIMES?What’s it all about?
• Completion time estimates can be estimated using the
Three Time EstimateThree Time Estimate approachapproach
• In this approach, three time estimates are required for each activity
4. MOST LIKELY TIME (m)
THREE TIME ESTIMATES
OPTIMISTIC TIME (a)
PESSIMISTIC TIME (b)
1
2
3
most frequently occur if the activity were
repeated many times
the shortest possible time to complete the
activity if everything went right
the longest possible time to complete the
activity if everything went wrong
5. with THREE TIME estimates, the activity completion
time
can be approximated by a BETA DISTRIBUTION
BETA DISTRIBUTION
BETA DISTRIBUTIONS CAN COME IN A VARIETY OF SHAPES :
6. FORMULA
MEAN AND VARIANCE
a + 4m + b
6
b - a
6
2
mean (expected time)
variance σ2
t
MOST LIKELY TIME (m)
OPTIMISTIC TIME (a)
PESSIMISTIC TIME (b)
12. EXAMPLEPROBABILISTIC ANALYSIS OF THE PROJECT NETWORK
ADD ALL THE σ2
OFF THE CRITICAL
PATH
S
D
what does this formula means?
what does these numbers mean?
14. EXAMPLEPROBABILISTIC ANALYSIS OF THE PROJECT NETWORK
25
WEEKS
EXPECTED TIME
μ
30
WEEKS
PROPOSED PROJECT TIME
X WHAT IS THE
PROBABILITY THAT
THE SYSTEM WILL BE
READY BY THAT
TIME?
WHAT IS THE
PROBABILITY THAT
THE SYSTEM WILL BE
READY BY THAT
TIME?
18. EXAMPLEACTIVITY EARLIEST AND LATEST TIMES AND SLACK
0.9719
probability of completing
the project in 30 days
0.9719
probability of completing
the project in 30 days
CPM : A single estimates for activity times
did not allow for any variation in activity times
Activity times were treated as if they were known for certain
PERT: multiple time estimates
allow for variation in activity times