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
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