{"id":"data2-explain-array-formula-teammate","audience":"analysts","category":"spreadsheets","track":"data","technique":"audience","level":"advanced","title":"Explain an AI-written formula to a teammate in three lines","free":false,"task":"A teammate pasted an AI-written lookup formula that shows blanks everywhere in your Excel 2016 workbook. Write exactly 3 plain-text lines labeled Does:, Breaks when: and Fix:, under 90 words, no code block. Fix must name a helper column key and say what to match on, with the formula for it written inline.","materialLabel":"Formula and sheets (attached to your prompt automatically)","material":"Teammate's message: \"The price column is all blank. The AI gave me this and it worked in its example.\"\n\nFormula in Orders!D2 (filled down 800 rows):\n=IFERROR(INDEX(Prices!C:C, MATCH(A2&B2, Prices!A:A&Prices!B:B, 0)), \"\")\n\nOrders tab:\nA SKU | B Size | C Qty | D Unit price\nHB-220 | L | 4 | (blank)\nHB-220 | S | 2 | (blank)\nAC-105 | M | 10 | (blank)\n\nPrices tab:\nA SKU | B Size | C Unit price\nHB-220 | S | 21.00\nHB-220 | L | 24.00\nAC-105 | M | 34.50\n\nNotes:\n- The workbook is used on Excel 2016 (no dynamic arrays). Formulas are entered normally, not with Ctrl+Shift+Enter.\n- The AI's example was run in a newer Excel where the same formula works.\n- IFERROR is there so errors don't show on the sheet.\n- Prices has 1,200 rows. SKU plus size is unique per row.","rules":[{"type":"plainText"},{"type":"maxWords","max":90}],"mistakes":[{"title":"Blaming the data","why":"The AI guesses typos or spaces instead of the array-entry problem.","fix":"Add: \"Explain it fails because Excel 2016 needs array entry for the combined range.\"","checks":["r0"]},{"title":"Not saying why blanks","why":"The AI misses that IFERROR hides the error.","fix":"Add: \"Say IFERROR turns the error into blanks.\"","checks":["r1"]},{"title":"Giving a long rewrite","why":"The AI posts the formula in a code block with extras.","fix":"Add: \"Fix: one sentence naming a helper column key, written inline, no code block.\"","checks":["r2"]},{"title":"Leaving the format to chance","why":"The AI picks its own length and layout unless told the exact shape.","fix":"Add: \"Plain text, no bold or headings.\"","checks":["plainText"]},{"title":"Trusting the AI to count words","why":"AI is bad at counting. Ask for a total word limit and it often lands over it.","fix":"Cap each part instead, for example \"each line under 15 words.\"","checks":["maxWords"]}],"promptHints":[{"label":"Asks for three labeled lines","patterns":["does:","breaks when:","fix:"]},{"label":"Names the version","patterns":["excel 2016","array","ctrl"]},{"label":"Mentions iferror","patterns":["iferror","blanks"]},{"label":"Asks for the helper column","patterns":["helper column","key"]},{"label":"Rules out code","patterns":["no code block","inline"]}],"hint":"Explaining a failing formula means tying the cause to the version and the symptom, then giving one concrete fix.","mission":"Why is this formula showing blanks?","musts":["Exactly 3 labeled lines","Name the Excel 2016 cause","Give the helper-column fix"],"rubricLabels":["Does: says the formula looks up the unit price by matching SKU and size together (A2&B2 against the combined SKU and Size columns)","Breaks when: says in Excel 2016 the combined range Prices!A:A&Prices!B:B needs array entry (Ctrl+Shift+Enter) or dynamic arrays, so it fails, and IFERROR hides the error as blanks","Fix: adds a helper column in Prices (for example D2 =A2&\"|\"&B2 or A2&B2) and matches A2&B2 against that column with a normal INDEX/MATCH, and there is no code block"]}