Which thirty invoices should you actually check?
Two hundred and forty invoices, one quarter, thirty to review. Periodic sampling spreads the thirty evenly across the ledger; random sampling draws thirty times and hands you twenty-seven different invoices, and knowing why is the point of this template.
Finance Starter Sampling free
After you install, this is the model to open.
Pull a Fair Sample of Invoices to Check
- 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 same ledger, sampled both ways:
- Periodic, every 8th
- 30 invoices, 10008 to 10240
- Random, 30 draws
- 27 different invoices
- Repeated in the draw
- 3 invoices came out twice
- Reproducible
- yes with the seed recorded
Random sampling here draws with replacement, so duplicates are the expected consequence rather than a fault: if you need thirty distinct invoices, draw forty, remove duplicates and take the first thirty. Periodic sampling has the quieter failure. If your ledger repeats on a cycle that shares a factor with your period, every draw lands in the same position in that cycle, so on a ledger where one person raises every fifth invoice, a period of five samples that person and nobody else. Neither method knows anything about money: both treat a $380 invoice and a $22,400 invoice as equally worth checking, so sample high-value items in full and run this on the remainder.
The model
A quarter of consecutively numbered invoices with the client and the person who raised each one. Sampling takes the identifier column and returns the sample it drew, so the draw itself is auditable.
| Invoices in the ledger | 240, numbered consecutively |
| Period | 8 (every eighth invoice) |
| Random draw | 30 samples, seed 4021 |
| Columns the ledger carries | client, raiser, amount |
| The sample drawn | written to its own sheet |
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.
Next question
- Which forecast should you actually quote?What Will Next Quarter's Fees Be?
- Is that client really slower or just one late invoice?Do These Two Clients Pay at the Same Speed?
- Is the plan limited by money or by people?Split a Fixed Budget Across Three Things
- How often does an ordinary year lose money?What If the Biggest Client Leaves?
- Did that payment land on the right client?Match the Invoices to the Client List
- What are the odds the carry is zero?What Does Carry Actually Look Like?
Every model like this one, and the method behind them: Statistics in Google Sheets.