[CQ-Contest] Re: Polling

[Dave_Hoaglin at abtassoc.com]

Pete, N4ZR, has usefully reminded us that it does not take a large sample
to get a reasonable margin of error.  (For a margin of error of + or - 3
percentage points, though, the sample size is more like 1100+.)  I would
like to offer some further comments, from the perspective of a statistician
who does some work with surveys.

The size of the sample does not scale with the size of the population.
That is, what matters is the absolute size of the sample, not what
percentage it is of the population (as long as the size of the population
is at least 20 times the size of the sample).

Thus, a scientific poll of QST readers would need responses from somewhat
more than 1100 readers to give a margin of error of 3 percentage points.

A key point is the way in which the sample is obtained.  The margin of
error that we've been quoting requires a random sample.  Not any old sample
will be random (I'm avoiding "representative" because it is a loaded word).
Getting a sample that is reliably random can take a lot of work.  That is
where the professional polling organizations come in.

Putting a survey up on a web site and inviting people to read it and
respond will almost surely produce a "sample" that is far from random.
With no control over the sampling mechanism, it will not be possible to
give any margin of error.  Worse yet, it will not be possible to say how
the "sample" is related to the population, even though it may be much
larger than the 1100 or mentioned above.  The "information" from such an
activity should not be taken seriously.

73, Dave K1HT


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