Determination Of Sample Size From Prevalence Study

Before You Input Values Please Remember

The Formula Used for this type of sample size determination is 

 Sample Size = ZP (1-P)/d2

Z – Statistical constant (1.96)

P – Prevalence  (that can be obtained from previous studies or a    pilot study).

d – Precision/ allowable error (corresponding to effect size) 


1. Select The study in Which The prevalence of Disease is 15-20%. As You know for the same disease some study reports the prevalence to be 15, some study will report 30% and even some study will report more than that. So its ideal that you select a study in which the prevalence of the disease is between 15-20%. Since you will have to give refence of that particular study so do not change the prevalence. Put exactly as reported in the study. Selecting a particular study is your choice but you will have to put prevalence exactly as per study.

While putting values in percentage put values in percentage like this

if prevalence is 10% then put = 0.1

if prevalence is 15% then put = 0.15

if prevalence is 17% then put = 0.17

so on and so forth

2. Precision /Allowable error depends upon prevalence of study but usually a value of 0.10 is used. The other values which can be used are 0.25,0.50,1.0 and 1.25 but if you dont want to complicate the things then keep it 0.1.

Formula Calculation

Don`t copy text!


There are many methods of sample size determination. It is one of the first hurdle when someone starts writing a thesis. I have tried to give simplest way of determination of sample size. You need to show the method to your PG teacher before you include this method in your thesis. First confirm from your PG teacher and then only proceed.

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