Last verified 23 September 2026. Power Query is built into Excel for Windows, Mac and the web. An Excel sheet holds at most 1,048,576 rows. KNIME is free. Claude's code execution works on every plan, including Free. Details for experts at the end of this book.

The idea in one page

Never clean the file; clean the process. A hand-cleaned file has no memory. Next month you, or someone else, makes slightly different choices, and the totals stop matching. A recorded process keeps the raw export untouched, applies the same rules every time, and writes a new clean table. When a rule is wrong you fix it once, and every month after is fixed.

Five kinds of mess, five rules. Almost every export breaks in the same places. Each one needs a written rule before it needs a formula:

Some of these rules aren't yours to make. Whether 01/08 is January or August depends on the instrument's settings. Whether two identical rows are one measurement or two real repeats is a question for the person who ran them. Write your guess, then ask.

The assistant finds, the query repeats. An assistant with code execution is very good at the first pass. It reads the export and lists every odd header, date format, unit and placeholder, with row numbers. It's the wrong place for the monthly cleaning: asked again next month, it may choose differently, and a colleague can't see its steps. So you use it to find the problems and draft a tricky formula, and the recorded Power Query steps do the monthly work.

Every row goes somewhere. Rows are never quietly dropped. Each raw row ends up in one of three places: the clean table, a flagged list with a reason (unknown unit, date it can't read, a conflicting duplicate), or the count of exact duplicates removed. The check block on the first sheet shows:

plus one control figure, such as the sum of the valid values, that the data owner can compare with their own system. If the line doesn't add up, nobody uses the table until someone looks.

Test it on someone else. The cleaning belongs to the team only when a colleague can drop next month's file into the folder, click Refresh, read the check block and explain a flagged row, without calling you.

Worked example: two exports, four quadrants

The four quadrants from T01-L01 · From Idea to Working Result: The Four-Quadrant Method, for an instrument export and then a finance export.

Lab: the monthly instrument export

Structure it. Clara, the PhD student, wants to stop being the only one who can clean the file before she leaves for her conference. "Every month the HPLC exports one CSV of about 400 rows. The group needs a clean table in mg/L for the group meeting. A useful result: a workbook in the group drive that anyone can refresh, that never loses a row silently, and that the PI, our group leader, can trust."

Understand it. She takes the August export, which holds no personal data, and uploads it to Claude with code execution on. Her request: "Don't clean anything yet. List every problem in this file: header rows, date formats, units, placeholder words, duplicate rows, with row numbers and a count for each. Tell me how many data rows you read." The answer lists the three notes rows above the header, three date formats, two units, 11 rows saying missing and 4 pairs of identical rows. It read 412 data rows, and her own count in Excel agrees.

Two findings stop her. Claude read 01/08/2026 as 1 August, which is right if the instrument writes day first, but she has never checked. And one of the "duplicates" is two rows with the same sample ID and slightly different values. Last year she deleted the second one each time. A year of her cleaning rests on decisions that were never hers to make. Back to Structure it: "A rules table agreed with Tomás, who runs the instrument. The query applies it. Anything the rules don't cover is flagged, not fixed."

Tomás settles it in ten minutes. The instrument writes dates day first. Identical rows are a known export glitch: keep one. Same ID with different values means a re-injection, and both stay, flagged for the analyst to choose. The word missing means the run failed. Clara writes this onto a Rules sheet: header on row 4, three date formats (day first), target unit mg/L (mg/L × 1, g/L × 1000), missing stays empty and is marked failed run, exact duplicates removed, same-ID conflicts flagged.

Choose tools. Power Query in Excel, because the whole group has Excel, the workbook lives in the group drive, and the recorded steps are visible to anyone who opens the editor. The alternative is KNIME, free and better for large files and many sources. At 400 rows a month, with colleagues who've never installed it, it would only add work. Claude stays in the process for one job: drafting a formula she can't write herself, which she then checks.

Build it. She points the workbook at the export folder with Data → Get Data → From File → From Folder, picks the folder and chooses Combine & Transform Data, so next month's file only needs to land there. The Power Query Editor opens, the window where the steps are recorded. She removes the three notes rows (Home → Remove Rows → Remove Top Rows), makes the next row the header (Use First Row as Headers), renames the columns by double-clicking their names to sample_id, measured_at and result, and splits result into number and unit at the space (Split Column → By Delimiter). The unit conversion is the one step she asks Claude for. A custom column (Add Column → Custom Column) is a new column filled by a formula: "Write a Power Query custom column: if unit is mg/L, value × 1; if g/L, value × 1000; otherwise null." Null means empty. She pastes it in and checks three rows by hand: 0.011 g/L becomes 11.0, 12.0 mg/L stays 12.0, and a µg/L test row becomes empty. Each step appears in the Applied Steps list on the right, with a name like Convert g/L to mg/L.

Then the routing, where she again asks Claude for each step she can't build and checks it. Rows with an unknown unit, an unreadable date or a same-ID conflict go to a Flagged sheet with a reason column. Exact duplicates are removed and counted. The clicks are in Details for experts. The first sheet shows the check block: 412 rows in = 396 clean + 12 flagged + 4 duplicates removed, and the sum of the valid values. She checks the sum against a pivot table, Excel's tool for totalling by group, on the raw file, filtered to the same rows. It matches.

The test: while she's still at her desk, her colleague Ben copies the September export into the folder and clicks Refresh all. The check block reads 418 = 399 + 15 + 4. Three rows are flagged unknown unit: the instrument wrote ug/L after a software update. Ben finds it in the Flagged sheet in a minute. Nothing was converted wrongly. Clara adds ug/L × 0.001 to the rules and the query, after asking Tomás.

What it taught her: the cleaning took an afternoon to build. Most of the value came from the ten minutes with Tomás.

Company: two department exports, one clean table

Structure it. Deniz, a team assistant in finance, prepares the table the controller uses each month. "Two department exports, about 300 rows each, into one table in euros for the monthly cost review. A useful result: a table the controller can reconcile with the booking system, refreshable by my deputy."

Understand it. Deniz uploads the August exports to the assistant the company has approved, with the employee names column removed first, and asks for the same problem list. Amounts come in two formats, EUR and kEUR (thousands of euros); 18 rows say pending; West writes dates as 02.08.2026. And one row, claim C-204, appears twice in West's file. Deniz asks the booking system's admin. It's a known double export when a claim is edited: keep one. Then the finding that changes the problem: the controller's 75-euro difference last month was exactly this row, deleted in July and kept in August. Back to Structure it: "The table has to reconcile with the booking system every month, and the reconciliation has to be on the page, not in my head."

Choose tools. Power Query again: the controller works in Excel, the deputy does too, and the workbook sits in the finance team's SharePoint library, Microsoft's shared document storage. Deniz considers asking the assistant to produce the clean table every month and rejects it. The controller would have no way to see what changed between months.

Build it. One query per department, both with the same named steps, stacked into one table. Rules sheet: kEUR × 1000; pending stays empty and is marked open; exact duplicates removed and counted; any other currency flagged. The check block shows rows in, clean, flagged and removed for each department, and the total in euros. The controller adds one line of her own: the total from the booking system, typed in each month, and the difference next to it.

The test: Deniz goes on holiday in September. The deputy refreshes, sees difference 0.00, and one flagged row with USD from a conference fee. She forwards that row to the controller instead of guessing a rate.

What it taught Deniz: the controller never doubted the arithmetic. She doubted where the rows went.

What goes wrong

Cleaning the file by hand. Symptom: this month's table is right, and nobody can say what was changed. Fix: keep the raw export untouched and do every change as a named step in the query.

Rows that disappear. Symptom: the clean table has fewer rows and no one knows why. Fix: route every row to the clean table, the flagged sheet or the duplicate count, and show the check line on the first sheet.

A placeholder turned into zero. Symptom: missing or pending becomes 0 and pulls the mean or total down. Fix: keep them empty, give them a status, and count them in the check block.

Guessing a rule that belongs to someone else. Symptom: you've been reading 01/08 as August for a year and the instrument writes month first. Fix: write your guess on the rules sheet and have the data owner confirm it. Put their name and the date next to it.

Letting the assistant do the monthly cleaning. Symptom: two months cleaned by the same prompt handle a duplicate differently. Fix: use the assistant to find problems and draft one formula at a time. Recorded steps do the repeat work.

A new unit converted silently. Symptom: after a software update a value is suddenly a thousand times too small. Fix: convert only the units on the rules sheet and flag everything else. A new unit should stop the table, not slip through it.

A workbook only you can refresh. Symptom: the query points at C:\Users\clara\Downloads. Fix: point it at the shared folder, and have a colleague refresh it once while you're still there.

Do it yourself (60 minutes)

1. Structure it (10 min). Write down the export, who gets the clean table, how often, and what they use it for. Name the person who owns the data.
2. Understand it (15 min). Upload one month's export, if you're allowed, and ask for a problem list with row numbers and a row count, without cleaning. Recount the rows yourself. Write the rules table and mark each rule you guessed. Ask the data owner about those.
3. Choose tools (5 min). Power Query if the team works in Excel and the files fit. KNIME if they don't. Write one line on why, and where the workbook will live.
4. Build it (25 min). Import from the shared folder, add each rule as a named step, route rows to clean or flagged, and build the check block with one control figure. Check three converted rows by hand.
5. Test it (5 min). Have a colleague drop the next month's file into the folder and refresh. Ask them to explain one flagged row from the workbook alone.

Exit check

Hand in a shared workbook that cleans one export, with:

• a Rules sheet: header, date formats, units and factors, placeholders, duplicates, and who confirmed each rule;
• the query, with named steps and the raw file untouched;
• a Flagged sheet with a reason for each row;
• a check block: rows in, clean, flagged, duplicates removed, and one control figure;
• the result of a refresh by a colleague on a second month's file.

It passes when rows in equal clean rows plus flagged rows plus duplicates removed, for both months, and the colleague can explain one flagged row from the workbook alone.

Rule to remember

Clean the process, not the file, and let every row show where it went.

Details for experts

Checked on 23 September 2026. You don't need this to follow the book; it's for setting things up or answering a colleague.

Further reading and tools

Next books

• T10-L01 · Ask a Spreadsheet a Question and Check Which Rows It Used: the row accounting this book builds on.
• T10-L03 · Build an Analysis Your Colleague Can Rerun and Get the Same Numbers: the next step, when the analysis after the cleaning has to be rebuilt by someone else.
• T12-L01 · Decide What You May Paste Into AI at Work: before any export goes into an assistant.
• T05-L01 · Your First Automation: Build It, Break It, Switch It Off: when the file should land in the folder on its own.

Tools

• Excel Power Query: Taught. The recorded, refreshable cleaning steps. Built into Excel, no separate page.
• Claude: Taught. Finds the problems in a sample export and drafts one formula at a time.
• KNIME: Catalogued. Free visual workflows for larger files or many sources.
• ChatGPT: Catalogued. Its data analysis finds problems in an export the same way.