creating a situation where users are separated into two different groups and then making a single variable different for each groups can indicate the effects of

creating a situation where users are separated into two different groups and then making a single variable different for each groups can indicate the effects of the variable given a large enough group size but taking that experience and applying it to history where many things are changing and where you cannot separate everyone into 2 different groups will not necessarily tell you anything about one arbitrarily selected variable.