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Element or unit: a single case in the population.
Population: all cases in which a researcher is interested.
Sampling frame: the list of elements from which the sample will be selected.
Sample: the elements (subset of a population) selected for investigation.
Representative sample: a sample that contains the same essential
characteristics as the population.
Probability sample: a sample selected using a random process so that each
element in the population has a known likelihood of being selected.
Non-probability sample: a sample selected using a non-random method.
Sampling error: the error that occurs because of differences between the
characteristics of the sample and those of the population.
Non-response: when an element selected for the sample does not supply the
Census: data that comes from an attempt to collect information from all elements
in the population.
Sampling error difference between sample and population.
A biased sample does not represent population. Some groups are over-
represented; others are under-represented. sources of bias non-probability
sampling, inadequate sample frame, non-response. Probability sampling reduces
sampling error and allows for inferential statistics.
Kinsey et al (1948) Sexual Behavior in the Human
Alfred Kinsey was a sexologist who suspected that there was a greater diversity of
sexual behaviour in the USA than had so far been acknowledged. He set out to
investigate this by collecting the personal narratives of 18,000 men (and later a
sample of women), inviting them to write about their sexual life histories. There
were two main stages of recruitment in this study. At first, Kinsey was content to
use a Sth 232
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non-probability, snowball sample of volunteers, including students, prison
inmates, colleagues and members of gay clubs, who would put him in touch with
other people they knew. He justified this on the grounds that sexuality was a
private and sensitive issue for which random
probability sampling would be inappropriate: he could not really approach
strangers on the street and ask them to provide an honest and detailed account of
their sexual experiences! Later, Kinsey was criticised for using ‘unscientific’
methods, for it was argued that his sample was biased towards those who were
forthcoming enough to volunteer their personal stories: indeed, these people did
report a higher level of sexual activity than those in his second sample. The latter
was what Kinsey called a
multistage cluster sample of ‘100% samples’, although O’Connell Davidson &
Layder (1994) argue that this technique was not rigorous enough to be a
probability sample. Kinsey had identified geographical clusters across the USA
and divided these into further sub-clusters; he then broke these down into ‘cells’
representing different social groups, stratified by age, residence, religious
affiliation and so on. He aimed to c