How many responses do you really need? This simple question is a never-ending quandary for researchers. A larger sample can yield more accurate results — but excessive responses can be very costly.
Consequential research requires an understanding of the statistics that drive sample size decisions.
Before you can calculate a sample size, we need to determine a few things about the target population and the sample you need:
- Population Size — how many total people fit your demographic? For instance, if you want to know about mothers living in the US, your population size would be the total number of mothers living in the US. Don’t worry if you are unsure about this number. It is common for the population to be unknown or approximated.
- The confidence interval (also called margin of error) is the plus-or-minus figure usually reported in newspaper or television opinion poll results. For example, if you use a confidence interval of 4 and 47% percent of your sample picks an answer you can be “sure” that if you had asked the question of the entire relevant population between 43% (47-4) and 51% (47+4) would have picked that answer.
- The confidence level tells you how sure you can be. It is expressed as a percentage and represents how often the true percentage of the population who would pick an answer lies within the confidence interval. The 95% confidence level means you can be 95% certain; the 99% confidence level means you can be 99% certain. Most researchers use the 95% confidence level.
- Standard of Deviation — How much variance do you expect in your responses? Since we haven’t actually administered our survey yet, the safe decision is to use .5 – this is the most forgiving number and ensures that your sample will be large enough.
There are three factors that determine the size of the confidence interval for a given confidence level:
- Sample size
- Population size
There are number of online solutions to determine your population size based on the confidence level.
Refer the link : http://www.surveysystem.com/sscalc.htm#one
The chart below gives an idea of number of respondents required as per the population size.
Determining the right sample size for your research