clean-data-xls

SKILL.md

Clean Data

Clean messy data in the active sheet or a specified range.

Environment

If running inside Excel (Office Add-in / Office JS): Use Office JS directly (Excel.run(async (context) => {...})). Read via range.values, write helper-column formulas via range.formulas = [["=TRIM(A2)"]]. The in-place vs helper-column decision still applies.

If operating on a standalone .xlsx file: Use Python/openpyxl.

Workflow

Step 1: Scope

If a range is given (e.g. A1:F200), use it

Otherwise use the full used range of the active sheet

Profile each column: detect its dominant type (text / number / date) and identify outliers

Step 2: Detect issues

Issue
What to look for

Whitespace
leading/trailing spaces, double spaces

Casing
inconsistent casing in categorical columns (usa / USA / Usa)

Number-as-text
numeric values stored as text; stray $, ,, % in number cells

Dates
mixed formats in the same column (3/8/26, 2026-03-08, March 8 2026)

Duplicates
exact-duplicate rows and near-duplicates (case/whitespace differences)

Blanks
empty cells in otherwise-populated columns

Mixed types
a column that's 98% numbers but has 3 text entries

Encoding
mojibake (é, ’), non-printing characters

Errors
#REF!, #N/A, #VALUE!, #DIV/0!

Step 3: Propose fixes

Show a summary table before changing anything:

Column
Issue
Count
Proposed Fix

Step 4: Apply

Prefer formulas over hardcoded cleaned values — where the cleaned output can be expressed as a formula (e.g. =TRIM(A2), =VALUE(SUBSTITUTE(B2,"$","")), =UPPER(C2), =DATEVALUE(D2)), write the formula in an adjacent helper column rather than computing the result in Python and overwriting the original. This keeps the transformation transparent and auditable.

Only overwrite in place with computed values when the user explicitly asks for it, or when no sensible formula equivalent exists (e.g. encoding/mojibake repair)

For destructive operations (removing duplicates, filling blanks, overwriting originals), confirm with the user first

After each category of fix (whitespace → casing → number conversion → dates → dedup), show the user a sample of what changed and get confirmation before moving to the next category

Report a before/after summary of what changed