[kgwgk]

You may not understand what "random sample of the overall population" means if you would expect every single person in the sample to be healthy enough to travel on a cruise. Almost 1% of the population in the US has Alzheimer disease or other dementias, for example.

[giantDinosaur]

I noted exactly that in my comment. Yes, not all the population can go on a cruise. No, it's not like it's only the healthiest 20% of the population that can. If I randomly sampled the population I'd expect the majority to be capable of a cruise. What do you think cruises are like?

[Klinky]

It's been hitting those in assisted living facilities rather hard, and those people would likely not be able to take a cruise.

[kgwgk]

If you agree that they are not a random sample of the population - they are healthy enough to be fit for travel (like the majority of the population) you are not trying to contradict ant6n's comment as I thought. I misunderstood, my apologies.

[rvnx]

and it's not a small travel (e.g. the British tourists that went on board the Diamond Princess).

If you are not in good health at the beginning, you don't adventure yourself 10 hours+ from your home. So this group is likely in better shape than average population.