This sample size calculator can help you calculate the sample size which represents how many people you need to interview in order to obtain results that reflect the population targeted, as precisely as you want. There is more info on this topic below the form.

## How does this sample size calculator work?

This statistics tool allows you determine both the sample size and the confidence interval when planning your survey, thus you will find there 2 tabs each one being designated to a specific calculation as detailed here:

- First tab “Find Sample Size” takes account of these variables that should be provided:

- Confidence level is known as confidence coefficient as well and represents the level of certainty expressed in percentage, that you assume when you calculate the required population sample size. The most used confidence levels are: 90%, 95%, 98% and 99%. What is important to note is that each of these most used confidence levels have their standard Z value: confidence level of 90% has Z score of 1.645, 95% has 1.96, 98% has 2.326 while 99% has a Z value of 2.576. Thus a higher confidence level results in a larger sample size to be interviewed.

- Confidence interval is known as the margin of error and indicates the percentage in plus-or-minus for the amount of error that you can tolerate. A safe confidence interval is set to be between 3 and 5 %.

- Population refers to the population you target to study or research. The sample size is a number of subjects that you need to interview and get the conclusions that are representative for the entire population.

- Pick certain choice % is the percentage for picking a certain choice. In other words the pick certain choice % is the response distribution.

- Second tab “Find Confidence Interval” requires these figures that should be known:

- Confidence level;

- Sample size;

- Pick certain choice %.

## What is the sample size formula?

The algorithm behind this sample size calculator uses the following formulas:

- For infinite/not specified population:

- For finite/specified population:

Where:

SS(infinite) is the sample size for a not specified population.

SS(finite) is the sample size for a specific population.

Z score for the confidence level chosen.

Pcp is the Pick certain choice percentage.

c is the confidence interval (margin of error).

15 Apr, 2015