Determining Sample Size Page 2 - Tarleton
Determining Sample SizePage 2Figure of Means for Of VariabilityThe third criterion, thedegree of variabilityin theattributes being measured refers to the distribution ofattributes in the population. The more heterogeneousa population, the larger the Sample size required toobtain a given level of precision. The less variable(more homogeneous) a population, the smaller thesample size. Note that a proportion of 50% indicatesa greater level of variability than either 20% or 80%.This is because 20% and 80% indicate that a largemajority do not or do, respectively, have the attributeof interest. Because a proportion of .5 indicates themaximum variability in a population, it is often usedin Determining a more conservative Sample size, thatis, the Sample size may be larger than if the truevariability of the population attribute were FOR DETERMININGSAMPLE SIZEThere are several approaches to Determining thesample size.
encouraged to adopt a new practice. Assume there is a large population but that we do not know the variability in the proportion that will adopt the practice; therefore, assume p=.5 (maximum variability). Furthermore, suppose we desire a 95% confidence level and ±5% precision. The resulting sample size is demonstrated in Equation 2.
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