{"id":"data2-ab-sample-runway-finish-date","audience":"product-managers","category":"data","track":"data","technique":"verification","level":"intermediate","title":"Work out when an A/B test will reach its sample size","free":false,"task":"Your VP asks when the checkout test can be called. Write exactly 4 labeled lines in plain text, under 80 words, with the math shown: Needed, So far, Remaining and Finish date. Count the start day as day 1, assume traffic stays the same every day, and give the finish date as the last day of the test.","materialLabel":"Test status (attached to your prompt automatically)","material":"Checkout test - status on the evening of Friday, Nov 7 (Bluebird Outdoor Gear)\n\nStarted: Monday, Nov 3 (counted as day 1).\nPlan: each version needs 8,000 visitors before the test can be called.\nTraffic: each version gets 400 visitors per day, split evenly, every day of the week including weekends.\nVisitors so far per version: 2,000 (days 1 to 5, Mon Nov 3 to Fri Nov 7).\n\nOther facts:\n- A sale on Nov 14 is expected to raise traffic, but the plan assumes the normal 400 per day and the sale is not counted on.\n- Early results look 0.4 points better for the new version, which is not part of this question.\n- The VP asked: \"When can we call it?\"\n- Calendar: Nov 8 is a Saturday. November has 30 days.","rules":[{"type":"plainText"},{"type":"maxWords","max":80}],"mistakes":[{"title":"Dividing the wrong numbers","why":"The AI divides 8,000 by 400 and forgets the 2,000 already collected.","fix":"Add: \"Remaining: 8,000 - 2,000 = 6,000, then 6,000 / 400 = 15 days.\"","checks":["r0"]},{"title":"Off-by-one date","why":"The AI counts from Friday or double counts the start day.","fix":"Add: \"Count the 15 days from Saturday Nov 8; the last day is Saturday Nov 22.\"","checks":["r1"]},{"title":"Adding the sale","why":"The AI shortens the test because of the Nov 14 sale.","fix":"Add: \"Ignore the sale; assume 400 visitors per day.\"","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 the labels","patterns":["needed:","so far:","remaining:","finish date:"]},{"label":"Shows the subtraction","patterns":["8,?000 ?- ?2,?000","6,?000"]},{"label":"Gives the day count","patterns":["15","/ ?400"]},{"label":"Says where to start counting","patterns":["nov 8","saturday","day 1"]},{"label":"Rules out the sale","patterns":["ignore the sale","sale"]}],"hint":"Date math needs a visible chain: needed, done, remaining, days, date. Tell the AI where counting starts.","mission":"When will this A/B test finish?","musts":["Four labeled lines","Math shown","Correct finish date"],"rubricLabels":["Needed states 8,000 visitors per version, and So far states 2,000 per version after 5 days","Remaining shows 8,000 - 2,000 = 6,000 visitors and 6,000 / 400 = 15 more days","Finish date is Saturday Nov 22 (15 more days after Friday Nov 7, or 20 days in all), and the reply is exactly four lines labeled Needed:, So far:, Remaining: and Finish date:"]}