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Inference on Proportions. Assumptions: SRS Normal distribution np > 10 & n(1-p) > 10 Population is at least 10n.

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Presentation on theme: "Inference on Proportions. Assumptions: SRS Normal distribution np > 10 & n(1-p) > 10 Population is at least 10n."— Presentation transcript:

1 Inference on Proportions

2 Assumptions: SRS Normal distribution np > 10 & n(1-p) > 10 Population is at least 10n

3 Formula for Confidence interval: Normal curve Note: For confidence intervals, we DO NOT know p – so we MUST substitute p-hat for p in both the SD & when checking assumptions.

4 A May 2000 Gallup Poll found that 38% of a random sample of 1012 adults said that they believe in ghosts. Find a 95% confidence interval for the true proportion of adults who believe in ghost.

5 Assumptions: Have an SRS of adults np =1012(.38) = & n(1-p) = 1012(.62) = Since both are greater than 10, the distribution can be approximated by a normal curve Population of adults is at least 10,120. We are 95% confident that the true proportion of adults who believe in ghost is between 35% and 41%. Step 1: check assumptions! Step 2: make calculations Step 3: conclusion in context

6 Another Gallop Poll istaken in order to measure the proportion of adults who approve of attempts to clone humans. What sample size is necessary to be within of the true proportion of adults who approve of attempts to clone humans with a 95% Confidence Interval? To find sample size: However, since we have not yet taken a sample, we do not know a p-hat (or p) to use!

7 What p-hat (p) do you use when trying to find the sample size for a given margin of error?.1(.9) =.09.2(.8) =.16.3(.7) =.21.4(.6) =.24.5(.5) =.25 By using.5 for p-hat, we are using the worst- case scenario and using the largest SD in our calculations.

8 Another Gallop Poll is taken in order to measure the proportion of adults who approve of attempts to clone humans. What sample size is necessary to be within of the true proportion of adults who approve of attempts to clone humans with a 95% Confidence Interval? Use p-hat =.5 Divide by 1.96 Square both sides Round up on sample size


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