Sample Size Calculator Case-Control Study

Based on odds ratio precision

Data Input: (Help) (Example)

Input

 

Results

α

 

P1

P2

n

OR

 

N

£`

 

 

 

Note:

Variables

Descriptions

£\

Probability of type I error

P1

Probability of exposure for people with the disease (Cases)

P2

Probability of exposure for people without the disease (Controls)

OR

Odds Ratio

£`

Odds ratio precision

n

Sample size for each exposure group

N

Required total sample size

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Application: This procedure is used to determine what sample size is needed to estimate the OR to within £` of OR with probability 1-£\.

Procedure:

  1. Enter

a)    Value of £\, the two-sided confidence level

b)    Probability of exposure for people without the disease (Controls)

c)    Odds Ratio

d)    Odds ratio precision

  1. Click the button ¡§Calculate¡¨ to obtain

a)    The probability of exposure for people with the disease (cases) and the sample size

  1. Click the button ¡§Reset¡¨ for another new calculation

Formulae:

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Examples

Example 1: What sample size would be needed in each of two groups for a case-control study to be 95% confident of estimating the population odds ratio to within 25% of the true value if this true value is 2.0, and the exposure rate among the controls is estimated to be 0.30?

P2 = 0.30

P1=(OR)*P2/[(OR)*P2+(1-P2)] = 0.4615

OR=2.0; £`=0.25

n = (1.96)2 * {[(1/(0.46*0.54))+1/(0.3*0.7)]}/[ln(1-0.25)]2= 408

Therefore, 408 subjects is required for each of the case-control group, which makes a total of 816 participants needed to be recruited.


Example 2: Using the above example, what should the minimum sample size be if we want the estimate to be within 50% of the true odds ratio?

P2 = 0.30

P1=(OR)*P2/[(OR)*P2+(1-P2)] = 0.4615

OR=2.0; £`=0.50

n = (1.96)2 * {[(1/(0.46*0.54))+1/(0.3*0.7)]}/[ln(1-0.50)]2= 71

Therefore, 71 subjects is required for each of the case-control group, which makes a total of 142 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.

Lwanga, S.K. and Lemeshow, S. Sample size determination in health studies: A practical manual. WHO, 1991. Print.

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