Sample size calculation : Comparative studies

Epomedicine
July 04, 2020
When case data is on nominal/ordinal scale and proportion is parameter of the study

n = [p(1-p1) + p2 (1 - p2)] / (p1 - p2)2 * C

n = Minimum sample size for a group

p1 and p2 = Proportion of two groups

C = Standard value for the corresponding level of α and β selected for the study:
  • 80% power and 95% CI = 7.85
  • 90% power and 95% CI =  10.51
  • 80% power and 99% CI = 11.68
  • 90% power and 99% CI = 14.88
When case data is on interval/ratio scale and mean is parameter of the study

n = [σ12 + σ22] (Z1-b + Z1-a/2)2 / d2

d = difference in means of two group (effect size)

σ1 = SD of Group 1

σ2 = SD of Group 2 

Z1-b = Desired power (0.84 for 80% power and 1.282 for 90% power)

Z1-a/2 = Critical value and a standard value for the corresponding level of confidence (1.96 at 95% CI and and 2.58 at 99% CI)

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