You’ve just landed a grant to study the economy of small Midwestern farming communities, and you choose the delightful little town of Cottonwood Falls, Kansas (pop. 903). But you only visit it AFTER it has been hit by a tornado.
Or you want to study the social structure of Japanese fishing villages – but you visit them exclusively the day AFTER they have been hit by tidal waves.
This might make sense if you are interested in the psychological effects of natural disasters. But if you want to see how a small Kansas farm town functions, or how people live in Japanese fishing villages, it’s a REALLY crappy study design.
Makes about as much sense as slurping up a jar of pond stuff, looking at a slide, and thinking you’ve seen how a freshwater ecosystem functions.