On 21 Nov 1999, KazakOR wrote:
> The city in which I live has a population of 126,635. I wish to sample
> this city and generate poll results with a margin of error of 3.5%.
> How many people must I contact to arrive at this confidence level?
Well, so far you haven't specified a confidence level. Do you wish to
attain this margin of error with the usual 95% confidence, or with some
other level? Secondly, the requisite sample size depends also on the
method of sampling proposed. Thirdly, it depends also on the value of
the true peercentage in the population. (I'm assuming that by "poll
results" you mean the percentage of persons who express a particular
opinion or otherwise answer a question in a particular way; "margin of
error of 3.5%" doesn't make a lot of sense otherwise.)
For 95% confidence, the sampling procedure used by the Gallup Poll in
1972, and percentages between 30% and 70%, a 3.5% margin of error would
require a sample of roughly 1000 persons. You might find George Gallup's
"The Sophisticated Poll Watcher's Guide" (Princeton Opinion Press, 1972)
of some interest.
BYW, the sample size does NOT depend on the size of the population being
sampled, unless the population is so small that the sample size is a
considerable fraction thereof. The same sample size would be required
for a population of 8 million as for 125,000. This point is somewhat
counterintuitive for most folks, who would be inclined to think (without
having thought about it too hard) that the precision of a result would
depend on the sampling fraction, not the sample size; and consequently
that a given level of precision would require a larger sample from the
8 million than from the 125,000. 'Tain't so. The precision of one's
result is a function of the quantity of information one has in hand, not
of the quantity of information in the population of interest.
> Thanks for any help! BTW, this is not homework; I'm 38 and am arguing
> with my statistics major wife who is probably right in the first place.
------------------------------------------------------------------------
Donald F. Burrill [EMAIL PROTECTED]
348 Hyde Hall, Plymouth State College, [EMAIL PROTECTED]
MSC #29, Plymouth, NH 03264 603-535-2597
184 Nashua Road, Bedford, NH 03110 603-471-7128
