PERT Standard Deviation
Standard deviation is a measure of uncertainty — in plain terms, how much "wiggle room" or risk is hidden inside a single estimate. A big standard deviation means the task is unpredictable; a small one means you're fairly confident it will land close to your estimate.
To calculate it, you take the pessimistic estimate ("P", the worst-case time or cost), subtract the optimistic estimate ("O", the best-case), and divide the gap by six. In other words, it's just the distance between your best and worst cases, spread out. The wider that gap, the more uncertain the task.
Reading the result: a small number (say, half a day) tells you the task is low-risk and easy to predict. A large number (say, several days) is a warning that this activity could swing a lot, so you may want a bigger schedule or budget buffer. Project managers use this to spot which tasks are the riskiest and to build realistic contingency into their plans.
Think of it like the difference between a weather forecast that says "between 70 and 72 degrees" versus one that says "between 40 and 90 degrees." The wider the gap between best and worst case, the less certain you can be about what actually happens.
For the webpage task, the optimistic estimate is 4 days and the pessimistic estimate is 14 days. Apply the formula: SD = (14 − 4) / 6 = 10 / 6 ≈ 1.67 days. So the standard deviation is about 1.67 days. That's a fairly wide spread, telling you this task carries real uncertainty — the actual duration could easily land more than a day off your 7-day estimate, so it's worth watching closely and padding your plan a bit.
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