Estimating Sample Size
October 16th, 2009Sampling is the process of selecting a subset of units from the population. After choosing a sampling method, the next step is to decide how many people should be sampled.
We use sampling formulas to determine how many to select. It is important to select the right amount of people because it is based on the characteristics of this sample that we make inferences about the population.
The most common method of sample size determination is based on proportions. For example, suppose we are preparing for the winter Olympics and are interested in estimating "the proportion of out of state skiers that took at least one overnight trip." We might use this number of people that would consider traveling to the Olympics.
In this case, the sample size is estimated using proportions. sp = (p/(1-p)/ n) where p is the proportion of "out of state skiers that took at least one overnight trip". The most conservative number for this proportion is .50 and if the desired accuracy was .05 and the formula would appear as: (Number of Standard Errors)2 * ((proportion)*(1-proportion)) / (Accuracy)
Using this formula explained above, the following sample sizes will result. If you are curious how your sample size will match up, take a look at this table.
Remember to carefully select your sample size to ensure a representative study.
You can see that as the sample size increases, the percentage of error decreases. Hope this explanation will help you choose the best sample for your research.
