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:
http://www.surveysystem.com/sscalc.htm
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 Ltd.
Tel: 08448 008 846
