Sample size calculation : Comparative studies
When case data is on nominal/ordinal scale and proportion is parameter of the study
n = [p1 (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)
Reference: http://njppp.com/fulltext/28-1567942207.pdf
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Epomedicine July 04, 2020
A researcher wants to compare the knowledge among Group A and Group B. From the previous study, proportion of the two groups is taken, i.e., 40% and 20%, respectively. A researcher wants 95% CI and 80% power for the study. Calculate the total subjects required for a new study.
p1 = 0.4
p2 = 0.2
C = 7.85
From calculation, n = 79 in each group
Epomedicine July 04, 2020
Assuming from the previous study that the mean (SD) of HbA1 c level in Group A and Group B was 5.5 (2.2) and 7.5 (2.0), respectively. At 95% CI and 80% power of the study, find out the sample size for the new study,
σ1 = 2.2
σ2 = 2
d = 7.5-5.5 = 2
From calculation, n = 153 in each group