Representativeness

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A fundamental component of any scientific data set is that it represents

some population of interest in a known way. The textbook scheme is to

randomly sample from a known population, but other well-defined samMEASURING

pling schemes are also used to draw inferences about known populations.

The NCVS uses a complex random sampling scheme. In Chapter 7, there is

a detailed discussion of case-control schemes that can be especially useful

for studying rare events like violence and crime.

Many of the data sets used to study firearms and violence are not

random samples from well-defined populations of interest, nor are they

exhaustive enumerations of any population. These types of data may provide

some information, as described above, but using them to assess the

effects of policy can be more complicated.