Cross-sectional studies: Longer duration of participant recruitment does not make them longitudinal

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.