What three-point estimate does your history give?
Forty finished tasks and six candidate shapes, ranked by how far each fitted curve strays from your actual data. Lognormal wins and the symmetric bell finishes a distant third, which is exactly the shape most plans assume.
Work Intermediate Statistics free
After you install, this is the model to open.
What Shape Are Our Task Durations?
- In your spreadsheet, click the Sortia icon in the strip of icons down the right-hand edge. No strip? Click the arrow at the bottom-right to open it. You can also use Extensions, then Sortia, then Open Sortia.
- Click Start from a template and put that name in the search box.
- Pick the card with that name and click Load this template. It arrives on a new tab with real numbers already in it.
The answer
How the six shapes ranked, and what the runner-up is for:
- Lognormal
- 0.0449 best fit
- Triangular
- 0.0835 second
- Normal
- 0.1420 a distant third
- Three-point estimate
- 1.29 / 2.85 / 18.21 days
These durations are not symmetric, and that is the finding: the most likely task is done in under three days while the average takes more than twice that, so a plan built on averages looks pessimistic to everyone on the team right up until the month it is late. The second-place fit is the one to carry forward, because Schedule Risk asks for exactly three points and 1.29, 2.85 and 18.21 are sitting in the triangular row of the report, taken from your own history. Then try it on subsets: two populations in one column is the most common reason a fit looks poor.
The model
One column of durations in days, with a three-point block beside it waiting for the answer. Distribution Fitting tests six shapes and ranks them smallest distance first.
| Tasks | 40, all finished |
| Measure | days from start to done |
| Shapes tested | 6 |
| Fitted mean / spread | 7.55 / 4.90 days |
| Best fit and its parameters | calculated |
Once it is in your sheet
- The model arrives with real numbers in it and runs as it stands, so you can press the button first and understand it second.
- Change the numbers to yours. The sheet marks which cells are inputs and which hold formulas, and most labels carry a note explaining the row.
- Press the run button at the bottom of the panel. It is labeled for the tool you are in, and the result lands on its own tab, with a written reading of it beside the figures.
Never used Google Sheets? Start here goes the whole way, in seven steps, and assumes nothing.
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Every model like this one, and the method behind them: Statistics in Google Sheets.