SSC Quickhelp

bullet1 Sample Size Calculation

Apart from corrections, there are three main factors that influence sample size:

  • Variability: the amount of difference within the population. If the population is very homogeneous with respect to the characteristic you want to investigate, you don't need a big sample size to establish a reasonable estimate of the characteristic. In statistical terms, this factor is measured by the standard deviation of the population characteristic (or a good estimate thereof).
  • Confidence: the amount of risk you are willing to take to reach false conclusions. This factor comes in two tastes. The first is to reach the conclusion that your research hypothesis is true, while in fact it is false. The second is the risk you take to say that the research hypothesis is false, while in fact it is true. These factors are statistically translated as alpha and beta. The smaller you want the risk to be, the higher a sample size you need.
  • Tolerance: How big a difference actually makes a difference for you? Suppose you have two mean values for a certain population characteristic: 10.2 and 10.23. Does this make sense, translated to your real world? Will you act differently for both groups? The smaller the difference you're interested in, the more samples you will need.

The way you need to interpret these three factors depends on the type of research you want to perform. The next section handles the subject of the analysis type.


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