Page 491 - Encyclopedia of Nursing Research

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of simple random sampling is that each case
Sampling has a known, nonzero probability of being
selected. This approach, however, is often
impractical and tedious and is not used
Sampling is a process for selecting a much. A more commonly used type of ran-
representative part of the population of dom sampling is systematic random sam-
interest so that one can make valid infer- pling. Systematic random sampling involves
ences and generalizations from the sample the use of a random start, and then the selec-
to the population. A sample is more feasi- tion of every kth case or incidence (e.g., every
ble, economical, and practical than using 5th, 10th, and 35th case). This approach is
the whole population. It also often is more more convenient than simple random sam-
accurate than trying to measure the entire pling, but it can have variance estimation
population. This is because the greater num- problems. A minimum of two systematic
ber of cases in a population, as compared random samples with independent ran-
with a sample, increases the likelihood of dom starts are needed to estimate variance,
nonsampling errors such as measurement unless one can assume a random distribu-
errors, nonresponse biases, and recording tion of the cases on the list from which one
and coding errors. Although many think has sampled.
of sampling representativeness in descrip- When using systematic random sam-
tive terms as only an issue of external pling, one must be very careful that the list
validity, or generalization, sampling also used does not have some systematic order
is concerned with the relationships found. or periodicity. If so, systematic random sam-
Therefore, sampling errors or biases may pling may lead to a seriously misrepresented
threaten the internal validity of studies sample or pattern. For example, one might
as well. Samples, however, are not techni- inadvertently select all nurse managers or
cally in and of themselves “representative,” obtain blood samples only when certain
“unbiased,” or “fair.” It is the sampling pro- hormones are at their peaks, if the sampling
cess that is representative, unbiased, or fair. interval mimics the sequencing of nurse
This is because we rarely if ever know the managers on the list or the time interval at
true population values and therefore can- which the hormone peaked.
not determine if any given sample is truly Stratified sampling is another type of
representative of the population. Rather, random sampling. It involves identifying
we rely on the assumptions underlying our one or more classification variables to use for
sampling process to make assertions about sampling purposes. With stratified sampling,
representativeness or bias. one randomly samples within each nonover-
There are several types of sampling. lapping strata of the classification variables.
Simple random sampling, or probability For example, if sex is the classification var-
sampling, is a procedure that may involve iable, then one randomly samples men and
the use of a table of random of numbers or women separately; if basic educational prep-
the flip of a coin to determine who or what aration of nurses is the classification variable,
will be included in the sample. A key feature then one randomly samples from those with