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# 9. Calculate samplesize for diagnostic study

Part 9 of 9; Calculate samplesize for diagnostic studies, sensitivity & specificity.

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### 9. Calculate samplesize for diagnostic study

1. 1. © Dr Azmi Mohd Tamil, 2012 Calculate Your Own Sample Size – Part 9 Diagnostic Study Jones SR, Carley S & Harrison M. An introduction to power and sample size estimation. Emergency Medical Journal 2003;20;453-458. 2003 1
2. 2. © Dr Azmi Mohd Tamil, 2012 Diagnostic Study Based upon literature review, identify the sensitivity & specificity of the diagnostic test being studied. Calculate the sample size based on the sensitivity using the following formula; 2
3. 3. © Dr Azmi Mohd Tamil, 2012 Terms TP = True Positive FN = False Negative SN = Sensitivity z = Confidence Interval normal distribution value i.e. for 95%, z = 1.96 P = prevalence of disease in the test population W = accuracy = 0.05 3
4. 4. © Dr Azmi Mohd Tamil, 2012 Example (SN=95%) SN = 95% z = 1.96 P = 30% W = 0.05 4
5. 5. © Dr Azmi Mohd Tamil, 2012 Example (SN=95%) 5
6. 6. © Dr Azmi Mohd Tamil, 2012 Diagnostic Study Calculate again the sample size based on the specificity using the following formula; 6
7. 7. © Dr Azmi Mohd Tamil, 2012 Example (SP=80%) SP = 80% z = 1.96 P = 30% W = 0.05 7
8. 8. © Dr Azmi Mohd Tamil, 2012 Example (SP=80%) 8
9. 9. © Dr Azmi Mohd Tamil, 2012 Diagnostic Study N for Sensitivity 95% = 243 N for Specificity 80% = 351 If interested in both sensitivity and specificity, then take the higher number (e.g. 351). 9
10. 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 10
11. 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. 11
12. 12. © Dr Azmi Mohd Tamil, 2012 References Jones SR, Carley S & Harrison M. An introduction to power and sample size estimation. Emergency Medical Journal 2003;20;453-458. 2003 12
13. 13. © Dr Azmi Mohd Tamil, 2012 THANK YOU 13