Pre-existing data are data that have been produced in the past for
some specific purpose. For example, the police and other authorities
keep data on the types and frequencies of the various categories of
crimes reported to them.
It can be costly to produce new data, so perhaps pre-existing data from
a previous specific study can be used to answer current questions.
Using available data, however, does require caution. Some reasons to
be cautious when using available data include:
The available data may have been drawn from a population which is
not identical with the population now under study.
The available data may have been chosen using a sampling method
not appropriate for the current study.
We may not be sure whether all the available data has been presented
for us – perhaps some was omitted from the previous study (possibly
even for valid reasons) and we don't know this.
The reporting of available data may be in categories which do not
exactly match the purposes of the current study.
Primary and secondary data
Available data can be classified as:
a) primary data, and
b) secondary data.
As far as possible, users of secondary data should check the integrity
of the quoted secondary data. Secondary data may have been used in a
selective way to suit the purposes of a study at hand. Unfortunately,
secondary data can even be used in a dishonest or inaccurate way not
consistent with the intent of the primary data.
