The PERT formula has exactly two inputs that trip candidates: the 4 and the 6. Most people get the three-point structure right. They add optimistic, most likely, and pessimistic. Where they lose points is the multiplication and the divisor, and not knowing when the exam wants the triangular average instead. PERT stands for Program Evaluation and Review Technique. On the exam, it means one thing: a weighted three-point estimate that gives extra weight to the most likely scenario. The formula: (O + 4M + P) / 6 O = Optimistic estimate The 4 reflects a core assumption: the most likely outcome is four times more probable than either extreme. The 6 in the denominator is the sum of those weights (1 + 4 + 1). You divide by 6 to get a single estimate. Example: A task has an optimistic estimate of 4 days, most likely of 7 days, and pessimistic of 16 days. PERT estimate = (4 + 4×7 + 16) / 6 = (4 + 28 + 16) / 6 = 48 / 6 = 8 days Without the 4, you get (4 + 7 + 16) / 3 = 9 days. That is the triangular average, which the exam calls a "simple average" or "triangular distribution." Two different answers, two different formulas. The question tells you which one to use. Both distributions appear on the PMP. The signal is in the question stem. Use PERT (beta distribution) when: The question says "three-point estimate" without naming a distribution The question asks for the "expected duration" and gives three estimates The question references a "weighted average of three-point estimates" Use triangular (simple average) when: The question explicitly says "triangular distribution" The question asks to "average the three estimates equally" If the question gives you three time estimates and asks for a duration with no distribution named, default to PERT. PMI treats PERT as the standard three-point method on the large majority of questions. PERT does not stop at a single estimate. It comes with two more formulas that appear on the exam. Standard Deviation (SD) = (P - O) / 6 This measures the spread of uncertainty around the PERT estimate. A wider spread means less confidence. Using the same example: SD = (16 - 4) / 6 = 12 / 6 = 2 days Variance = SD squared = [(P - O) / 6]^2 Variance = 2^2 = 4 days squared When do these show up? Schedule range questions. PMI may give you three tasks on a critical path and ask for the total standard deviation of the path. You add the variances (not the standard deviations), then take the square root. Path total variance = variance of task 1 + variance of task 2 + variance of task 3 Candidates who add standard deviations directly get the wrong answer. This shows up on practice exams as a "close but wrong" choice. The most common arithmetic error. A candidate in a rush writes (O + M + P) / 6 instead of (O + 4M + P) / 6. The 4M disappears and the answer is 1-2 days off. Before calculating, write "4M =" on your scratch paper. Force yourself to multiply before you add. A question asks: "What is the variance of this task's duration estimate?" Optimistic: 3 weeks. Most likely: 8 weeks. Pessimistic: 19 weeks. SD = (19 - 3) / 6 = 16 / 6 = 2.67 weeks If the answer choices include both 2.67 and 7.11, you need to know which the question asks for. Variance always gets squared. SD does not. The exam gives you a two or three-task critical path and asks for the total range of completion time. Wrong: add the standard deviations directly. 2 + 1.5 + 3 = 6.5 days. Right: add the variances, then take the square root. (4 + 2.25 + 9 = 15.25, then sqrt(15.25) = 3.9 days.) You do not need to know the statistics theory behind this. You need to remember: add variances, not standard deviations, then root the total. Your project has two tasks on the critical path. Task A: Optimistic 2 days, Most Likely 5 days, Pessimistic 14 days. Step 1: PERT estimates Task A = (2 + 4×5 + 14) / 6 = (2 + 20 + 14) / 6 = 36 / 6 = 6 days Step 2: Critical path duration 6 + 4 = 10 days total Step 3: Standard deviations Task A SD = (14 - 2) / 6 = 12 / 6 = 2 days Step 4: Variances Task A variance = 4 days squared Step 5: Path variance and path standard deviation Total path variance = 4 + 1 = 5 days squared Step 6: The range question If the question asks "In what range is there roughly a 68% chance the path finishes?" the answer is 10 ± 2.24 days, or 7.76 to 12.24 days. (68% probability corresponds to ±1 standard deviation. 95% is ±2 SD. 99.7% is ±3 SD. The exam uses these probabilities directly.) When you see three duration estimates, ask two questions before touching any math. Question 1: Does the question name a distribution? If it says "triangular," use (O + M + P) / 3. If it says "beta" or "PERT," or names nothing, use (O + 4M + P) / 6. Question 2: Does the question ask for an estimate, a standard deviation, or a variance? Estimate = use PERT directly. Standard deviation = (P - O) / 6. Variance = square the standard deviation. If it then asks about a path, add variances, not standard deviations, then take the square root for the path standard deviation. PERT questions are not a large chunk of the PMP exam. You will see 3-5 estimation questions total. But they are nearly always worth full marks or zero, because each question usually has one unambiguous correct answer and four plausible wrong answers. The candidates who miss these questions share a consistent pattern: they know the formula but not the decision rules. They can write (O + 4M + P) / 6 on demand, but under time pressure they add standard deviations across tasks or report variance instead of SD. That gap between recall and application shows up as a cluster on the PassCoach bias diagnostic. If your mock scores are inconsistent on quantitative questions, there is a good chance PERT is part of the pattern. Join the waitlist to get early access when the diagnostic launches. The formula is not the hard part. Knowing which number to report and when not to use it at all is where the points shift.