Large Counts Condition vs 10% Condition

Both terms below come up in the same part of the course, and students mix them up. Here is each one defined on its own, side by side, so you can see where they part company.

Large Counts condition

Sampling distributions

The Large Counts condition checks that np is at least 10 and n(1-p) is at least 10, so the sampling distribution of p-hat is close to normal.

Large Counts is a sample size check: you need roughly 10 successes and 10 failures before you can treat the sampling distribution of p^\hat{p} (p-hat, the sample proportion) as normal; that is the textbook name for it, and the AP course framework calls the same check the normality condition. Which value of pp goes into the check depends on the procedure. A significance test has a hypothesized value, so you verify np010np_0 \ge 10 and n(1p0)10n(1-p_0) \ge 10 using p0p_0 (p-naught); a confidence interval has no hypothesized value, so you verify np^10n\hat{p} \ge 10 and n(1p^)10n(1-\hat{p}) \ge 10, which is just counting the observed successes and failures. Testing p0=0.25p_0 = 0.25 with n=60n = 60 passes, since np0=15np_0 = 15 and n(1p0)=45n(1-p_0) = 45.

Full entry for large Counts condition

10% condition

Sampling distributions

The 10% condition says a sample drawn without replacement should be no more than 10% of the population, so the observations stay nearly independent.

Sampling without replacement makes each draw depend a little on the ones before it, because removing a person changes what is left in the population. The 10% condition, written n0.10Nn \le 0.10N (n at most one tenth of N, the population size), keeps that dependence small enough that standard deviation formulas built for independent draws still hold. In a school of 1,200 students, 0.10(1200)=1200.10(1200) = 120, so a sample of 80 satisfies the condition while a sample of 200 does not. You only need this check when sampling without replacement.

Full entry for 10% condition

Where each one fits in the course