Check out the great paper introducing the concept, here: doi.org/10.1093/aje/kw…
Target validity tells us about how well we have learned about that from our trial (or observational study).
What is the causal effect of using a parachute on 5-minute post-jump survival among the population of people who are about to jump out of an airplane, vs an empty backpack?
In the #PARACHUTETrial, internal validity depends on whether we have a good estimate of the effect of using parachute vs backpack *in our trial*
#PARACHUTETrial had perfect adherence & complete follow-up, so this is easy to estimate: the per-protocol effect is *equal* to the intention-to-treat effect
0/12 - 0/11 =0 percentage point risk difference of death at 5 minutes post-jump.
There’s no confounding and there’s no loss to follow-up, so this is probably the true value *for the study population* (although, as others have noted the sample size is quite small)
This asks us to assess whether the answer we got for our study population tells us anything about our target population!
In the #PARACHUTETrial, the target population is anyone about to jump out of any airplane.
They tried to recruit people currently on an airplane, but only succeeded in recruiting people who got onto a stationary grounded plane for the purpose of becoming eligible for the trial!
That’s different from the target pop!
There are 2 things to consider:
1️⃣is the effect the same for everyone?
2️⃣if it’s not, are types of people diff in these 2 pops?
1️⃣Do parachutes work same no matter speed & height of plane? No!! We definitely don’t think so!
2️⃣Do the types of people in 2 pops differ? Yes! We just said so!
Does #PARACHUTETrial tell us about the effect of a parachute (vs backpack) on 5-minute post-jump survival among people who are about to jump out of any airplane?
No! Target validity for this question is low!
Which is what makes it such a good case study.