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Confidence intervals for proportions: Type font preference

Plain type fonts such as Times New Roman are easier to read than fancy fonts such as Gigi. A group of 25 volunteer subjects read the same text in both fonts. (This is a matched pairs design. One sample procedures for proportions, like those for means, are used to analyze data from matched pair designs.) Of the 25 subjects, 17 said they preferred Times New Roman for Web use. But 20 said that Gigi was more attractive.

(a) Because the subjects were volunteers, conclusions from this sample can be challenged. Show that the sample size condition for the large-sample confidence interval is not met, but that the condition for the plus four interval is met.

(b) Give a 95% confidence interval for the proportion of all adults who prefer Times New Roman for web use.

(c) Give a 90% confidence interval for the proportion of all adults who think GIgi is more attractive.

Solution Summary

This solution includes detailed answers for each question. Provided is a detailed explanation of finding the plus four interval for proportions.