Day 8 · Use it on real work

Spreadsheets and numbers

Never trust an AI total again — and never need to, because you know the split.

13 min readPlus one task, about 10 min

The one idea

AI is good at the shape of a spreadsheet. It can turn a messy list into columns, suggest a formula, explain what a pivot table is and tell you which rows deserve attention. It is not a calculator you should trust merely because its answer has two decimal places.

The safe division of labour is plain. Let the model propose structure and code. Let a spreadsheet, a calculator or a second independent check perform the arithmetic. A formula is a transparent instruction that a machine can repeat. A number typed in a paragraph is only a claim until you have checked it.

Why it matters

A wrong number in a chat can be corrected and forgotten. A wrong number in a quotation, budget or stock sheet travels. It gets copied into an email, used in a decision and repeated by somebody who assumes the first total was checked.

The trap is that arithmetic mistakes are often small. A total of 9,842 instead of 9,924 looks reasonable. The tool does not put a nervous expression beside it. It writes the result in the same polished tone it uses for a correct total. The skill today is not distrust of every output. It is knowing which part can be delegated and which part must be executed by a system you can inspect.

The tools you will use

From messy list to checked number

1. Start by preserving the raw data. Make a copy of the original list and work on the copy. Do not let an AI clean away a row before you have a way to compare what went in with what came out.
2. Ask the assistant to identify fields and anomalies, not to announce a final answer. It may spot that dates use two formats or that one price has a currency symbol. You decide whether its interpretation is correct.
3. Ask for one formula at a time and name the exact range. A formula such as `=SUM(C2:C18)` is an instruction you can see and test; 'the total is 4,382' is only a result. Put the formula into the spreadsheet rather than copying the result.

   Give me a formula for Google Sheets or my spreadsheet app that totals the amount column.
   Use the exact column and row range I provide. Explain what each reference means.
   Do not calculate the total in your reply.

4. Run the formula in the spreadsheet, then test a second way. Select the cells and look at the status bar, use a calculator on a small sample, or make a second SUM on a copied range. Independent means a different method, not asking the same model twice.
5. Inspect edge cases: blank cells, negative amounts, duplicated rows, tax, currency and filtered-out rows. A correct formula can still answer the wrong business question if the range or definition is wrong.
6. Record the rule beside the sheet. Write what the total includes, what it leaves out and the date the data was checked. Numbers become safer when their meaning travels with them.

AI-generated illustration

See it

Suppose a shop keeps a rough list of sales in a message thread: "Monday bolts 12 at 4.50; Tuesday washers 8 at 3.25; Wednesday bolts 9 at 4.50". A chat assistant can turn that into Date, Item, Quantity and Unit price, and it can suggest a Revenue column with =C2*D2. That is useful structure. It has made the hidden calculation visible.

Now put the rows in Google Sheets and let the sheet calculate the revenue. If the assistant also says the total is 93.50, do not use that sentence as your control total. The spreadsheet formula should produce the number, and a calculator or an independent subtotal should agree. If they do not agree, stop and find the row, rather than choosing the most confident answer.

The same split applies to a budget. Ask AI to group expenses and suggest categories. Ask the spreadsheet to calculate totals. Ask yourself whether 'monthly cost' includes an annual fee divided by twelve. The hardest part is often not addition. It is agreeing what the column means.

Catch it making an arithmetic mistake

You need to see this one rather than be told it, and the method — three independent totals — is the habit this task is really teaching.

1. Take a genuinely messy list you already have: expenses, stock, contacts, invoices, a thread of prices, or ten to thirty lines of another useful record. Do not tidy it first. The mess is the point.
2. Paste it into your chosen assistant with this request. Save the original and the cleaned reply together, and remove private details first.

   Turn this into a clean table with sensible column headings. Then list the column headings only, and tell me what you would check first.
   Do not invent missing values. Mark unclear values as unclear.
   
   [PASTE YOUR LIST]

3. Move the cleaned table into a spreadsheet — Google Sheets is free and is where the next step works. Compare at least five rows with the original list before you calculate anything.
4. Ask for formulas, one at a time. Enter the formulas yourself and check that the range includes every intended row and no heading or total row twice.

   Give me a spreadsheet formula that totals the [amount] column.
   Then one that shows the average.
   Then one that highlights any row where [amount] is above [your threshold].

5. This is the key step. Ask the assistant to total the column itself, then compare that against the formula it gave you running in your spreadsheet, and against a calculator or a second independent subtotal. Write all three results in a note.

   Now add up the [amount] column yourself and tell me the total.
   List the rows you included. If a value is unclear, stop and ask rather than guessing.

6. Write down which of the three you now trust, in what order and why. Add one sentence describing what the amount column actually includes. Keep the sheet; it is your first checked data example, not disposable practice.

What you should notice

The formulas are right more often than the manual total. That distinction matters. A formula is a small program with visible inputs and repeatable behaviour; a paragraph containing a total is a claim produced by a text generator. When the manual total is wrong, it is usually wrong by a small, believable amount, which is the worst kind of wrong.

Notice that the assistant does not flag uncertainty. It does not say, 'I think this is about four thousand.' It says four thousand in the same tone it uses for things that are true. A warning triangle on a chart cannot fix that. The check has to sit in the workflow before the number travels.

Your rule from today is therefore not 'never use AI with numbers'. Let it build the structure, explain the data and write the formulas. Let the spreadsheet or calculator perform the arithmetic, and let you decide what each column means.

AI-generated illustration

If you have five more minutes

Give it numbers that sum to an obviously round figure, such as exactly 1,000. Ask for a mental total, then run the same numbers through a spreadsheet. If the reply is 998 or 1,004 without comment, you have seen the mechanism plainly. Do not use the result in a real decision.

Tomorrow

Research you can actually defend.

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