obg.cuhk.edu.hk 1 October 2016
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Sample size for comparing means of two Independent groups

How: Enter the number of groups, the anticipated within group standard deviation (assuming those from different groups are homogenous), and the anticipated differences or the minimal difference that is of interest, in the appropriate text boxes. Click Calculate button, and the results will show.
  Anticipated difference in means       
  Anticipated standard deviations
  Ratio Controls to Experiment Subjects

Reference: Independent Groups - Machin D, Campbell M, Fayers, P, Pinol A (1997) Sample Size Tables for Clinical Studies. Second Ed. Blackwell Science IBSN 0-86542-870-0 p. 24-25

Explanation: This calculation is used at the planning stage of a survey or an experiment, when the study is designed to test a null hypothesis whether the groups were similar or different.   The sample size is based on the anticipated or desired Effect Size (difference between group / standard deviation).  Sample size for each combination of Type I error and power is shown. 

Example: Say the study compared 2 groups, where the anticipated difference is 0.5, and the anticipated standard deviation is 1.0.  The sample size required for an Alpha of 0.05 and a power of 80% is 64 cases per group.

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