Sample Size Calculator: Cohort Study
Based on relative risk precision
Data Input: (Help) (Example)
Input
Results
α
P1
P2
n
RR
N
£`
Variables
Descriptions
£\
Probability of type I error
Proportion of the exposed group
Proportion of the non-exposed group
Relative Risk
Relative risk precision
Sample size for each exposure group
Required total sample size
Help Aids Top
Procedure:
a) Value of £\, the two-sided confidence level
b) Proportion for the exposed group
c) Relative Risk
d) Relative Risk precision
a) The exposed group proportion and the sample size
Formulae:
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Notations:
α The probability of type I error (significance level) is the probability of rejecting the true null hypothesis
P1 The proportion of the exposed group
P2 The proportion of the non-exposed group
RR The relative risk
£` The relative risk precision (within x% of the true RR)
n Required sample size for one of the exposure group
N Total sample size needed for the cohort study
Examples
Example 1: Suppose an outcome is present in 20% of the unexposed group of a cohort study, with 95% confidence, how large a sample is required for each of the exposed and unexposed groups in order to estimate the relative risk to within 10% of the true value of 1.75?
of the exposed and unexposed groups in order to estimate the relative risk to within 10% of the true value of 1.75?
P2 = 0.2
P1=(RR)*P2=1.75*0.2=.35
RR=1.75; £`=0.1
n = (1.96)2 * [(0.65/0.35)+(0.8/0.2)]/[ln(1-0.1)]2= 2027
Therefore, 2027 per group of participant is required, which makes a total of 4054 participants needed to be recruited.
Example 2: An epidemiologist is planning a study to investigate the possibility that a certain lung disease is linked with exposure to a recently identified air pollutant. What sample size would be needed in each of two groups, exposed and not exposed, if the epidemiologist wishes to estimate the relative risk to within 50% of the true value of 2.0 with 95% confidence? The disease is present in 20% of people who are not exposed to the air pollutant. P2 = 0.20 P1=(RR)*P2=2.0*0.2=0.40 RR=2.0; £`=0.5 n = (1.96)2 * [(0.60/0.40)+(0.80/0.20)]/[ln(1-0.5)]2= 44 Therefore, 44 per group of participant is required, which makes a total of 88 participants needed to be recruited. Reference: Lemeshow, S., Hosmer Jr., D.W., Klar, J.,and Lwanga S.K. Adequacy of Sample Size in Health Studies. WHO, 1990. Print.
pollutant. What sample size would be needed in each of two groups, exposed and not exposed, if the epidemiologist wishes to estimate the relative risk to within 50% of the true value of 2.0 with 95% confidence? The disease is present in 20% of people who are not exposed to the air pollutant.
within 50% of the true value of 2.0 with 95% confidence? The disease is present in 20% of people who are not exposed to the air pollutant.
P2 = 0.20
P1=(RR)*P2=2.0*0.2=0.40
RR=2.0; £`=0.5
n = (1.96)2 * [(0.60/0.40)+(0.80/0.20)]/[ln(1-0.5)]2= 44
Therefore, 44 per group of participant is required, which makes a total of 88 participants needed to be recruited.
Lwanga, S.K. and Lemeshow, S. Sample size determination in health studies: A practical manual. WHO, 1991. Print.