How many interviews? Decisions based on research require that it is based on robust data and statistically reliable.

  • Fieldwork Assistance can advise on how few interviews you can get away with
  • How reliable will my results be

The simple answer is – it depends on how reliable your data needs to be.  But you rarely need more than 400 interviews.

Test out possible interview numbers in our Calculator below

Sample Size Calculator

How many interviews? How robust the data?

Calculate Sample Size

Confidence Level: 95% 99%
Confidence Interval:
Population:
         
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Find Confidence Interval

Confidence Level: 95% 99%
Sample Size:
Population:
Percentage:
         
Confidence Interval:

You will need to know what the total population of your survey is – how many customers, how many businesses or how many residents in an area.

The confidence interval or margin of error, is the plus-or-minus figure usually reported in newspapers or on television.  Suppose 41% percent of your sample gives a particular answer.  If you have a confidence interval of 5, you can be certain that between 36% (41-5) and 46% (41+5) of your total population would have given the same answer.

The confidence level is a percentage measure of how sure you can be that your findings are accurate.  If your sample size conforms to the 95% confidence level, then 95 people in every 100 in your total population would answer the same way.  Most of our clients use 95%.  97.5% gives greater accuracy but can require a disproportionately larger sample.

When people are virtually unanimous, you can be sure of the answer, whatever the sample size.  When people are split more or less 50/50, the consequences of error may be much greater.  This is why market researchers assume the worst case, a 50/50 split, when calculating the sample size needed.

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