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Activity Number: 283
Type: Contributed
Date/Time: Tuesday, August 4, 2009 : 8:30 AM to 10:20 AM
Sponsor: Section on Statistical Education
Abstract - #305008
Title: T-Test for Proportions? Making Do When Your Software Can't Do Confidence Intervals for Proportions
Author(s): Gerald Shoultz*+
Companies: Grand Valley State University
Address: Department of Statistics, Allendale, MI, 49401,
Keywords: t-test ; z-test ; confidence intervals ; proportions ; SPSS
Abstract:

First statistics courses usually teach confidence intervals (CI's) and hypothesis tests (HTs) for one proportion and for the difference of two proportions using a z-test. Unfortunately, some computer packages (SPSS, SAS) do not have separate routines designed to find CI's and/or perform HTs for all these proportions cases, forcing students to do them by hand. A few websites and even the occasional statistics book improperly refer to a t-distribution for proportions. First, we point to examples of such websites and books. Second, for various sample sizes and sample proportions we compare margins of error for CI's for one proportion (z for proportions vs. t for means) and difference of two proportions (z for difference of proportions vs. non-pooled t for difference of means) and recommend sample size/sample proportion pairs for which the z and the t distributions give similar results.

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