HKSM
Three-Point Estimating
A method for estimating cost or duration by combining optimistic, most likely, and pessimistic values into a simple or weighted average, used when activity estimates are uncertain.
Key Points
- Captures uncertainty with three inputs: optimistic (O), most likely (M), and pessimistic (P).
- Two common expected-value formulas: Triangular = (O + M + P) / 3; PERT (weighted) = (O + 4M + P) / 6.
- Can quantify variability: standard deviation often taken as (P - O) / 6 and used to form ranges.
- Applies to activity cost, duration, or effort and is typically based on expert judgment and historical data.
Example
A task to configure a server has O = 4 days, M = 6 days, P = 12 days. Triangular estimate = (4 + 6 + 12) / 3 = 7.33 days. PERT estimate = (4 + 4*6 + 12) / 6 = 6.67 days. Standard deviation = (12 - 4) / 6 = 1.33 days, so a rough range is about 6.67 +/- 1.33 days.
PMP Example Question
Which estimating technique combines optimistic, most likely, and pessimistic values to produce an expected duration and can also indicate variability?
- Analogous estimating
- Parametric estimating
- Three-point estimating
- Reserve analysis
Correct Answer: C — Three-point estimating
Explanation: Three-point estimating uses O, M, and P to compute an expected value (triangular or PERT) and can provide a standard deviation to express uncertainty.
