NONE: ONLINE-ADS>>
ONLINE-ADS>>
(no name) ((no email))
(no date)
Date: Sat, 25 Apr 1998 13:02:08 -0700
To: online-ads_at_o-a.com
From: Bob Schaffer <schaffer_at_primenet.com>
Subject: Re: ONLINE-ADS>> Surveys about Email Conflict? Research validity?
Keith Pieper <kpieper_at_matchlogic.com> wrote:
>I hear 5% is a valid sample size? Is it? If so, are we so hung up on
>finding what works, that we'll swallow survey results regardless of how
>valid they are?
>Anyone add to proper sample size? Or better yet, how has email worked?
Keith:
There is no relationship between sample size and population size; all that
matters is variability and how sure we want to be that we don't make a
mistake. Think of it this way, if everyone in the population is identical
then we would only need a sample size of 1 -- it is only as people differ
that larger samples are needed.
As both a research consultant and full-time research educator, I find that
this simple counter-intuitive fact is very hard for many people to
internalize. Of course, if we plan to "cut" the data many ways: perhaps
by gender, income, geographic location, customer type, and so on -- then we
would need to be concerned with "cell size" -- the number of people in the
smallest demographic group (e.g., wealthy females who live in the NE and
buy frequently from outlet stores). Each statistical technique requires a
minimum number of people per cell (or average minimum) and it is easy to
have hundreds of cells requiring a very large sample size. Of course, when
people do differ, larger sample sizes, up to a point of diminishing
returns, will let us make more accurate estimates.
In actual practice, when dealing with the usual 5-point agree/disagree
Likert type scales, I have found a sample size of 200-400 (depending upon
the number of cells) to work very well and be cost effective.
Bob
--------------------------------------------------------
Dr. Robert W. Schaffer
Department of International Business and Marketing
California State Polytechnic University
3801 W. Temple Ave., Pomona, CA 91768
VOICE: (909) 869-2441 FAX: (909) 869-3647
rwschaffer_at_csupomona.edu
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