Re: Sample size - How big should it be?

Sun, 27 Jan 2008 08:41:02 GMT from Rob <user@xxxxxxxxxxx>:
Rob <user@xxxxxxxxxxx> wrote in news:TEEmj.7232$421.4228@news-
server.bigpond.net.au:

How do you decide how big your sample should be?

Ok I have k = N/n

OK lets say I was doing an employee survey total population 1000 (N)

Using a random number generator I pick a sample of (n)? employees to
survey.

Still not sure what k is meant to be.

How do I work out what size my sample I would need to get a
representative sample? What is considered to be significant?

Is this course work? If so, it looks like right now you're in the
sampling-and-descriptive-statistics portion of the course, and your
question will be answered in the inferential-statistics portion.

Briefly, "representative" and "significant" are matters of degree.
You have to decide up front what an acceptable significance level is.
(5% and 1% and 0.1% are common choices, but not the only ones.) Or if
you're trying to estimate a quantity, you decide up front what margin
of error you can accept and what level of confidence you need. Either
way, once those decisions are made then there's a formula that tells
you how big a sample you need.

(The formulas also require you to know something about your
population -- often this is obtained through a small pilot study.)

While I have you - if I don't use random sample ie. I invite everybody
in the company to take part in an anonymous online survey and only take
the people that reply. How do I determine if my results are meaningful?

That one's easy: they're not. All the statistical techniques you will
learn depend on having a good random sample. If you have a self-
selected sample, there's an excellent chance they are not
representative of the population.

I mean if 1% reply I would say the result do not mean much.

Not necessarily. Presidential polls typically sample about 1000
voters, which is well under 1% of all voters, yet they are accurate
to within ±3%. When testing yes/no data, what matters is how many are
in the sample and not what percent of the population is in the
sample.

If 100% reply then I could be confident of the response. How do I
determine what size sample is big enough.

Tell us more about what you're trying to measure, and what
significance level or confidence level you're shooting for.

--
Stan Brown, Oak Road Systems, Tompkins County, New York, USA
http://OakRoadSystems.com/
"If there's one thing I know, it's men. I ought to: it's
been my life work." -- Marie Dressler, in /Dinner at Eight/
.

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