Information Technology Reference
In-Depth Information
1. Shared exposure to the same physical or social environment
2. Self-selection in belonging to the group
3. Sharing of behaviors, ideas, or diseases among members of the
group
A sample consisting of the first few animals to be removed from a cage
will not satisfy these criteria either, because, depending on how we grab,
we are more likely to select more active or more passive animals. Activity
tends to be associated with higher levels of corticosteroids, and corticos-
teroids are associated with virtually every body function.
Sample bias is a danger in every research field. For example, Bothun
[1998] documents the many factors that can bias sample selection in
astronomical research.
To forestall sample bias in your studies, determine before you begin the
factors can affect the study outcome (gender and life style, for example).
Subdivide the population into strata (males, females, city dwellers, farmers)
and then draw separate samples from each stratum. Ideally, you would
assign a random number to each member of the stratum and let a com-
puter's random number generator determine which members are to be
included in the sample.
Surveys and Long-Term Studies
Being selected at random does not mean that an individual will be willing
to participate in a public opinion poll or some other survey. But if survey
results are to be representative of the population at large, then pollsters
must find some way to interview nonresponders as well. This difficulty is
only exacerbated in long-term studies, because subjects fail to return for
follow-up appointments and move without leaving a forwarding address.
Again, if the sample results are to be representative, some way must be
found to report on subsamples of the nonresponders and the dropouts.
AD HOC, POST HOC HYPOTHESES
Formulate and write down your hypotheses before you examine the data.
Patterns in data can suggest, but cannot confirm hypotheses unless these
hypotheses were formulated
before
the data were collected.
Everywhere we look, there are patterns. In fact, the harder we look,
the more patterns we see. Three rock stars die in a given year. Fold the
United States 20-dollar bill in just the right way and not only the
Pentagon but the Twin Towers in flames are revealed. It is natural for us
to want to attribute some underlying cause to these patterns. But those
who have studied the laws of probability tell us that more often than not
patterns are simply the result of random events.
