Checking your own work

By the end of this lesson you can re-check a count, a total and a chart before you share them.

Why it matters

Everyone makes mistakes. Professionals catch theirs before anyone else does. One wrong number in a report can make readers doubt all the right ones, so a few minutes of checking protects all your work.

The idea in plain words

A pilot walks around the plane before every flight, even after thousands of flights. Not because they expect a problem, but because a checklist catches the one they did not expect.

Before you share a data story, run five checks:

1. Re-count: check one number a second, different way (a filter and a formula, or by hand).
2. Totals: do the groups add up to the whole?
3. Units and period: does every number say what it measures (sales? items? naira?) and when?
4. Chart: does the axis start at 0? Is it the right type (bar or line)? Does it have a title and a source?
5. Words: does every sentence match a number you actually have?

Five checks, about five minutes. They catch most mistakes.

Check (cross-check): working out a result a second, different way to confirm it. If two methods agree, you can trust the number more.

See it done

Amara's nephew wrote a draft about her week. Before Amara shares it with her savings group, she asks you to check it.

The draft: "In the week of 2 to 7 March 2026, Amara's stall sold 249 fruits. Watermelon had 22 sales. 28 sales were paid in cash."

Data: market-stall-week.csv (synthetic).

1. List every claim. Three numbers: 249 fruits, 22 watermelon sales, 28 cash sales.
2. Check each a second way. In empty cells: - =SUM(F:F) gives 249. Correct. - =COUNTIF(E:E,"Watermelon") gives 13. Not 22! - =COUNTIF(H:H,"Cash") gives 28. Correct.
3. Find the cause of the mistake. =SUMIF(E:E,"Watermelon",F:F) gives 22. (SUMIF adds up only the matching rows: here, the quantity of every watermelon sale. You met it in lesson 3.5's "Go further" box.) So 22 is the number of watermelons (items), not sales. The draft mixed up rows and items, the trap from lesson 3.2.
4. Check the units and period. "Fruits", "sales", and the week are named. Good.
5. Fix the sentence. "Watermelon had 13 sales, in which 22 watermelons were sold."

One claim in three was wrong. The check took two minutes.

Your turn, with help

Check this draft about Lucía's survey: "All 30 pupils answered every question. 14 pupils come by bus."

Hint: for the first claim, think about blanks (lesson 2.5). Try =COUNTBLANK(A2:I31), which counts the empty cells in the whole table.

Your turn, alone

Check each claim with a formula. Write correct or wrong, and fix any wrong one.

1. "Amara made 12 sales on Friday."
2. "4 pupils in the survey have no siblings."
3. "Orange was Amara's best-selling fruit by items, with 103."
4. A chart shows Amara's sales per day with an axis from 7. Which check does it fail?

Watch out

• Checking with the same method. Re-running the same formula repeats the same mistake. Use a different way.
• Only checking the numbers you doubt. The mistakes are usually in the numbers you were sure about. Check them all.
• Changing the data to fit the story. If a check fails, change the story, never the data.

Say it back

1. Name the five checks.
2. In the draft, why was "22 sales" wrong?
3. Why check with a different method?

What is next

Your numbers are now right. But a correct number can still be stretched too far: "Lagos shoppers love cash!" Next lesson: how to say exactly what you found, and no more.