Posted by
Art Kendall on
Jun 12, 2011; 3:57pm
URL: http://spssx-discussion.165.s1.nabble.com/Margin-of-error-tp4481843p4481965.html
The finite population correction is used only in the
context where the pop under consideration is all that there is any
projection or generalization to. Often the intent is to project
and generalize to some kind of future or super-population.
I would guess that the source of the numbers did not apply the fpc.
If the DV is dichotomous, the width of the margin of error depend on
how close the estimate is to 50%.
Are the DV's the same in the three scenarios? I.e., are you
reporting the MOE as a percent of the estimate?
<soapbox on>
Most people, including sampling text book authors I have spoken to,
cannot cognitively (i.e., without doing some calculation) compare
"margins of error" at different confidence levels. Unless there is
a special sub-discipline where it is conventional to use 90%, I
recommend using 95%. For most purposes in designing a study, I find
that client decision makers can deal with effect size. So I
recommend leaving power and confidence level at the conventional 80%
and 95% and design to detect a given effect size. In addition many
people who have had just a smattering of exposure to statistical
thinking have the idea that using 90% is a sign of fudging or even
deliberate disinformation.
<soapbox off>
Art Kendall
Social Research Consultants
On 6/12/2011 10:52 AM, Michelle Tan wrote:
Hi Everyone
i have a question on margin of error.
Let's take an example:
at 90% precision, the margin of error (MOE) is given as below where N
denotes pop size and n sample size
N1 = 1000, n1=221, Margin of error = 1.52%
N2=500, n2=200, margin of error = 4.5%
N3=71, n3=67, Margin of error = 5.45%
The 3rd scenarios having the highest MOE, would mean that the findings
drawn from sample are less precise. Is that right? but if i were to look
at the sample size, no doubt it's the smallest among the 3 samples but if
i were to look at the pop size, i am already sampling 94% of pop. wouldn't
the variability within this sample be smaller compared to first/second
sample?
it just doesn't sound right when i am telling my client that sample 3 has
the largest sampling error and thus less precise?
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Art Kendall
Social Research Consultants