Beyond Common Core
Problem 44 of 45s12-044
Students by year and how they get to school. Juniors: bus 26, car 18, walk 16. Seniors: bus 24, car 42, walk 24. For a test of independence, find the expected count of juniors who come by car, and the df.
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Solution walkthrough2:00
- Add up the table: 60 juniors and 90 seniors; 50 bus, 60 car, 40 walk; 150 students in all.
- If year and travel are independent, juniors make up the same share of every column: .
- Expected = row total column total grand total .
- df : once two cells are known, the totals fix the rest.
- Traps: 18 is the observed count, df forgets to subtract 1, and 25 with df treats it as goodness of fit.