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 Longitudinal v prospective study?  

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What is the difference between a longitudinal study that follows subjects forward in time and a prospective study, which does the same?

For example why is the following a longitudinal rather than a prospective study?

"In a study, a group of people are exposed to an environmental toxin but are not treated. Instead, they are observed over time on a standard set of measures to ascertain the potential effects of the toxin."

Is it because it involves an intervention at the very beginning (the environmental toxin exposure) while a prospective study doesn't involve any intervention at all?

I'm confused here sad
Any replies will be much appreciated smiling face


First, remember horizontal and longitudinal. Horiz means from left to right or vice-versa. Longit means from top to bottom or reverse.

There are 2 studies – Experimental and Observational.

Longitudinal study is an OBSERVATIONAL only and it is done by Cohorting.

Cohort has 2 methods:
1. PROSPECTIVE – when you start counting the cases from present time (today) to onwards (till tomorrow or uncertain FUTURE times)
2. RETROSPECTIVE- when you start searching ( &collecting) the cases from today to PREVIOUS days ; i.e. PASSED time cases

In other words: all Prospective studies are longitudinal studies but longitudinal studies are NOT ONLY Prospective studies! Prosp study itself is a Longi but Longi can be also a Retrosp.

For your specific question:
Longi does multiple observations for EACH INDIVIDUAL over a certain period of time.
Prosp does multiple observations for a GROUP OF PEOPLE over a certain period of time.

Actually, there is no much difference between these two terminologies; and I think one can use both. But the term longitudinal is more specific than other.

This is what I know about it.

PS.!I want to know from other colleagues whether I am right as well.

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