Cochran Formula 1977. If sampling is proportional and the variances in all strata have same value S we obtain the simple result. However since this sample size exceeds 5 of the population 16790584 Cochrans 1977 correction formula should be used to calculate the final sample size.
Formula For Calculating A Sample For Proportions For populations that are large Cochran 196375 developed the Equation 1 to yield a representative sample for proportions. The variance reduces to Vy I s2Nn 1 I wsh2 58 Corollary 3. May 19 2009 Cochran 1977 has given a modus operandi for sample size determination.
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Download Full PDF Package. Rule of thumb is that you want at least 30 samples to be statistically reasonable so yes a sample size of 18 is likely to be too small with a population of 800. However since this sample size exceeds 5 of the population 16790584 Cochrans 1977 correction formula should be used to calculate the final sample size. STAT30143914 Applied Statistics-Sampling Preliminary References 1Cochran WG.
