For those reading at home, the meaning is that the &quot;most likely&quot; is multiplied by 4, giving a triangular distribution that (very very) roughly approximates a normal distribution.<p>I went hunting for the origins of that formula a few years ago. I couldn&#x27;t find the original source, and certainly none of the sources I found had a <i>justification</i> for it.<p>Someone who works at RAND could probably pull the original internal work and tell us. But I suspect it was chosen for ease of calculation.<p>While it&#x27;s been criticised for being normal-esque instead of pessimistically skewed, it still outperforms single-point estimations. I think it&#x27;s because of the &quot;unpacking effect&quot; that a full PERT estimate causes you to undergo.<p>There&#x27;s a literature of people tinkering with the formula, but I don&#x27;t think it&#x27;s anywhere near as important as the unpacking effect is.