Sometimes researchers think that they
are doing a longitudinal study simply because their study requires a longer
duration to recruit the needed number of study participants. However, it is not
the length of time it takes to enroll the needed number of study participants
that defines cross-sectional studies. It is the temporal relationship of the
ascertainment of exposure and outcome that matters.
In cross-sectional studies exposure and
outcome are ascertained at the same time. That is both are measured at the same
point in time. One doesn’t precede the other. Measurement per study participant
is done only once. No repeat/follow up measurement at some later time.
Conversely, the distinguishing feature
of longitudinal studies is repeated measurement. Two or more measurements are
done per study participant at given time intervals. The temporal relationship
between exposure and outcome is such that the outcome comes later than the
exposure.
To conclude, a cross-sectional study may take a year to accomplish
whereas a longitudinal study may be accomplished in few weeks or months. They
can simply be distinguished by the fact that the former involves a single
measurement per study participant (no repeat/follow up measurement at a later
time) and ascertainment of exposure and outcome is done at the same time while
the latter involves at least one baseline measurement and at least one other
follow up measurement and the presumed exposure is ascertained before the
outcome occurs.
