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# 6. Calculate samplesize for cohort studies

## on Sep 21, 2012

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Part 6 of 9; Calculate samplesize for cohort studies, dichotomous outcome

Part 6 of 9; Calculate samplesize for cohort studies, dichotomous outcome

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## 6. Calculate samplesize for cohort studiesPresentation Transcript

• © Dr Azmi Mohd Tamil, 2012 Calculate Your Own Sample Size – Part 6 Cohort Study 1
• © Dr Azmi Mohd Tamil, 2012 Cohort In a cohort study, you identify those who are currently disease-free. Among them, you identify those with and without the exposure/risk factor. Then this cohort is followed up for a pre-determined amount of time to identify those who developed the disease and those who didn’t. For example you want to prove that overweight adults have higher risk of diabetes mellitus compared to normal weight adults. 2
• © Dr Azmi Mohd Tamil, 2012 Example – overweight have higher risk of DM From literature review, identify the rate of disease among those with & without the risk factor. Ratio of unexposed vs exposed; 1:1 Proportion of sample from no-risk (Normal) population = 50% Proportion of sample from at-risk (Overweight) population = 50% P1=true proportion of DM in no-risk (Normal) population = 7% P2=true proportion of DM in at-risk (Overweight) population =32% 3
• © Dr Azmi Mohd Tamil, 2012 From Literature Review: Obesity & Diabetes M. DM + (32%) Overweight Sample DM - (68%) ratio (1:1) DM + (7%) Normal DM - (93%) Rifas-Shiman SL et al, 2008.Diabetes and lipid screening among patients in primary care: A cohort study. BMC Health Services Research. 4
• © Dr Azmi Mohd Tamil, 2012 Calculate Manually Calculate using these formulas (Fleiss JL. 1981. pp. 44-45) m=n1=size of sample from population 1 n2=size of sample from population 2 P1=proportion of disease in population 1 P2=proportion of disease in population 2 α= "Significance” = 0.05 β=chance of not detecting a difference = 0.2 1-β = Power = 0.8 r = n2/n1 = ratio of cases to controls P = (P1+rP2)/(r+1) Q = 1-P. n1 = m n2 = rm From table A.2 in Fleiss; If 1- α is 0.95 then cα/2 is 1.960 If 1- β is 0.80 then c1-beta is -0.842 5
• © Dr Azmi Mohd Tamil, 2012 Calculate Manually 6
• Or Use StatCalc© Dr Azmi Mohd Tamil, 2012 So you’ll need a sample size of 46 each for both groups. Total of 92. 7
• © Dr Azmi Mohd Tamil, 2012 Or use PS2 So the sample size required for each group is 38. Total of 76 StatCalc = 92 vs PS2 = 76 8
• © Dr Azmi Mohd Tamil, 2012 PS2 We are planning a study of independent cases and controls with 1 control(s) per case. Prior data indicate that the failure rate among controls is 0.07. If the true failure rate for experimental subjects is 0.32, we will need to study 38 experimental subjects and 38 control subjects to be able to reject the null hypothesis that the failure rates for experimental and control subjects are equal with probability (power) 0.8. The Type I error probability associated with this test of this null hypothesis is 0.05. We will use an uncorrected chi-squared statistic to evaluate this null hypothesis. 9
• © Dr Azmi Mohd Tamil, 2012 PS2 (translated) We are planning a cohort study with a ratio of 1 unexposed to 1 exposed. According to a previous study (Rifas-Shiman SL et al, 2008) indicate that the rate of DM among normal weight subjects is 7%. The rate for overweight subjects is 32%. Therefore we will need to study 38 overweight subjects and 38 normal weight subjects to be able to reject the null hypothesis that the DM rates for overweight and normal weight subjects are equal with probability (power) 0.8. The Type I error probability associated with this test of this null hypothesis is 0.05. We will use an uncorrected chi-squared statistic to evaluate this null hypothesis. 10
• © Dr Azmi Mohd Tamil, 2012 What If There Is No Prior Information? Instead of saying "Sample sizes are not provided because there is no prior information on which to base them“, do this instead;  Find previously published information  Conduct small pre-study  If a very preliminary pilot study, sample size calculations not usually necessary 11
• © Dr Azmi Mohd Tamil, 2012 Conclusion You can calculate your own sample size. Tools are available and most of them are free. Decide what is your study design and choose the appropriate method to calculate the sample size. If despite following ALL these notes fastidiously, your proposal is still rejected by the committee due to sample size, kindly SEE THEM, not us. 12
• © Dr Azmi Mohd Tamil, 2012 References (incl. for StatCalc) Fleiss JL. Statistical methods for rates and proportions. New York: John Wiley and Sons, 1981. Gehan EA. Clinical Trials in Cancer Research. Environmental Health Perspectives Vol. 32, pp. 3148, 1979. Jones SR, Carley S & Harrison M. An introduction to power and sample size estimation. Emergency Medical Journal 2003;20;453-458. 2003 Kish L. Survey sampling. John Wiley & Sons, N.Y., 1965. Krejcie, R.V. & Morgan, D.W. (1970). Determining sample size for research activities. Educational & Psychological Measurement, 30, 607-610. Snedecor GW, Cochran WG. 1989. Statistical Methods. 8th Ed. Ames: Iowa State Press. 13
• © Dr Azmi Mohd Tamil, 2012 References (PS2) Dupont WD, Plummer WD, Jr: Power and Sample Size Calculations: A Review and Computer Program. Controlled Clinical Trials 11:116-128, 1990 Dupont WD, Plummer WD, Jr: Power and Sample Size Calculations for Studies Involving Linear Regression. Controlled Clinical Trials 19:589-601, 1998 Schoenfeld DA, Richter JR: Nomograms for calculating the number of patients needed for a clinical trial with survival as an endpoint. Biometrics 38:163-170, 1982 Pearson ES, Hartley HO: Biometrika Tables for Statisticians Vol. I 3rd Ed. Cambridge: Cambridge University Press, 1970 Schlesselman JJ: Case-Control Studies: Design, Conduct, Analysis. New York: Oxford University Press, 1982 Casagrande JT, Pike MC, Smith PG: An improved approximate formula for calculating sample sizes for comparing two binomial distributions. Biometrics 34:483-486, 1978 Dupont WD: Power calculations for matched case-control studies. Biometrics 44:1157-1168, 1988 Fleiss JL. Statistical methods for rates and proportions. New York: John Wiley and Sons, 1981. 14
• © Dr Azmi Mohd Tamil, 2012 THANK YOU 15