What is Probability?Let’s start with something off-topic. Say you have developed a computer program that guesses a person’s age based upon his/her first name. Behind the scene, the program is so crude that all it does is to randomly pick a number between 1 to 100 and spits it out.
Assume that I’m a user of your program. What’s the probability that your program will guess my age correctly? Since the draw is completely random and total number of outcomes is one hundred, the probability of the computer picking any number is one in hundred (1/100 or 0.01) or simply 1%. So, the probability of your program to guess my age correctly is only 1%.
Estimate in a RangeObviously you don’t score a lot of points with your friends when you demonstrate this program to them. It seldom guesses anyone’s age correctly. After a bit of humiliation, you decide to overhaul the program and build in more intelligence. You modify the program such that instead of giving a single number, it gives the age in a range of ten. Instead of saying, “Your age is 27 years”, it now says “Your age is between 21 and 30 years”.
With this modification, what have you done? You have made a 10-fold increase in the accuracy of your program. Now the program has a one in ten (or 10/100 or 0.1) or 10% probability of guessing a person’s age (or the range) correctly.
What did you learn here? When making an estimate (or a guess), it better to estimate in a range, instead of a single value. By doing that you have a better chance to be correct.
Estimation in ProjectsLet’s switch back to Project Management now. As I said in the beginning, an estimate by definition is a guess. Assume that I’m a project manager and you are one of my team members. I ask you to give me an estimate for an activity, which you are not very familiar with. You think you can complete the activity in 8 days, but you are not quite sure. In order to save your back, you ‘pad’ another 8 days to the estimate, and give me an estimate of 16 days.
Do you know that you just violated the PMI Code of Ethics and Professional Conduct. What? How? Padding is inherently evil, to the extent of being unethical. Yes, unethical. It not just results in overestimation and increased project costs, it also pollutes the historical data, which is used on future projects.
Trivia: Do you know the difference between a pad and a reserve?
3-point EstimatesWhy should we do something unethical when there’s an easier and ethical alternative. Instead of you telling me that the activity would take 16 days to complete, why don’t you be transparent and tell me the truth? You could say that this activity would take 8 days, but you are not 100% sure. In the worst case, this activity might even take 16 days. Whereas, if things work out really well, you might be able to complete it in 6 days too. So, your best case estimate is 6 days, most likely estimate is 8 days, and worst case estimate is 16 days.
- O (Optimistic) = 6 days
- M (Most Likely) = 8 days
- P (Pessimistic) = 16 days
Summary of Key Points on Project Estimation
- One time estimate gives too much opportunity for a person to pad the estimate.
- Padding the estimates is unethical
- In order to get a sense of uncertainty or risk in the estimate, it's best to estimate in a range.
- By estimating in a range, we are able to get a more realistic estimate.
Please feel free to post your questions, comments or suggestions.
Full 9-part series on Project Estimation and PERT
- Get Intimate with PERT
- The Power of Three in Project Estimation (you are here)
- Three-point estimates vs PERT - What's the difference?
- Say Hello to PERT
- The Magical Formula of PERT
- Probability and Statistics in Project Management
- PERT and CPM get Cozy
- PMP Quiz Contest - Activity Duration Estimates
- Standard Deviation and Project Duration Estimates
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