Crew B is 16% ahead. Is that the crew, or the fortnight?
Three crews on the same detail, eight days each, and averages of 61.50, 67.75 and 58.50 square meters a day. Everyone on site already knows who is fastest. The test says the evidence does not.
Construction Starter Statistics free
After you install, this is the model to open.
Do the Three Crews Work at the Same Rate?
- 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
The gap is real on the sheet and not yet real in the statistics:
- p-value
- 0.1132
- F
- 2.42 against a critical 3.47
- Variance inside a crew
- 63 to 81
- Days each
- 8 not enough to see through it
Three crews of identical ability would produce a spread this big about eleven times in a hundred, so this evidence does not establish that the crews differ at all. That is a harder result to accept than a significant one and on a real site it is the more common one. The reason is visible in the numbers: one crew's own day-to-day swing is larger than the gap between crews. The fix is not a better test, it is more days, so rerun after another month of diaries before you move anybody. ANOVA answers whether the group averages differ at all, never which pair does.
The model
One crew per column with the crew name in the header row and eight daily outputs underneath. Single-factor ANOVA compares the three averages against the spread inside each crew.
| Crews | 3 |
| Days each | 8 |
| Measure | square meters a day |
| Averages | 61.50 / 67.75 / 58.50 |
| F, critical value and p | 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.
The method behind this one, worked end to end: ANOVA in Google Sheets.