The folks of the Theory of Constraints Center have a little project game to demonstrate how task and resource dependencies combine with a bit of unpredictability to create a cascade effect.
Imagine you have a simple project with five tasks, each of which is expected to take seven days. Since Tasks A and B can be done in parallel, and Tasks C and D can be done in parallel, we should expect the whole project to be done in 21 days.
But those five tasks don’t take exactly seven days each — they take roughly seven days each — and for our little game we roll a pair of dice for each task to represent that uncertainty.
Let’s say our random task durations come out as follows:
Task A — 5 days
Task B — 9
Task C — 3
Task D — 8
Task E — 6
Then our whole project is done in 23 days, not 21.
Hey, that’s not so bad, right? You roll high sometimes; you roll low sometimes. We expected 21, but it came out 23.
But let’s take another look. Our average roll was just 6.2 — less than 7. We rolled better than average, yet our total was worse than average. In fact, if we look at each color — I’m assuming each color represents a different resource dependency — then each color scores better than average too.
Delays accumulate, while advances do not.