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Sample Sizes

From time to time we are asked by clients; how many visits or calls they should make to give a robust sample of mystery shopping events for the entire population (of say sites or people that answer the phone).

There are some "market researchy" terms that require a little explanation:

Sample Size Terminology

The confidence interval 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.

When you put the confidence level and the confidence interval together, you can say that you are 95% sure that the true percentage of the population is between 43% and 51%.

The wider the confidence interval you are willing to accept, the more certain you can be that the whole population answers would be within that range. For example, if you asked a sample of 1000 people in a city which brand of cola they preferred, and 60% said Brand A, you can be very certain that between 40 and 80% of all the people in the city actually do prefer that brand, but you cannot be so sure that between 59 and 61% of the people in the city prefer the brand.

So, did you get all that?

If you're happy with the way that all that sounds and you want to calculate your own sample size before you get in touch with us (and do bear in mind that we may find a way around things that allow you to get good granularity of data and robust results for less!) try this site:

It's great and easy to use but do so with an open mind, as we said, these calculators are great if you have a wide ranging and large scale programme but if you have just 3 retail outlets with 1 staff in total, it may be far easier to just give us a call.

Impact Mystery Shopping

Tel. +44 (0) 7884 161461



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