In analyzing a situation using data, one of the common errors that one can fall into is that of sampling error, when one bases one’s conclusion on a sample that is not representative of the population at large. That is how many stereotypes and prejudices arise, because people form judgments about entire groups based on their experiences with just a few members of that group that they happen to encounter in their own lives.
Researchers are of course aware of this pitfall and they take steps to take this into account, either by making sure the sample is properly drawn or, if necessary, deliberately taking a sample that is not representative and making weighted adjustments after the fact to take this into account. [Thanks to Crip Dyke in comment #4 for the link-MS} For example, when taking opinion polls, subgroups that make up only a small fraction of the total population are over-sampled so that that subgroups’ sizes are larger and the views are more representative of the subgroups and not distorted by a few outliers. Then when those results are folded into the total sample results, adjustments are made accordingly. Those who do not understand this process sometimes seize upon the over-sampling of small groups to denounce the poll results as having been deliberately skewed to favor those groups.
Even though I am aware of this pitfall, I still succumb to it on occasion in making casual conclusions without thinking things through. The sheltering in place requirements imposed due to the current pandemic brought this home to me. I live in a condominium complex that has about a hundred units. Soon after moving here last August, I came to the conclusion that it must consist mostly of older people and retirees because they were the ones I usually saw whenever I went outside, which was not often since I tend to live mostly a quiet life at home.
That was the source of my error. Because I am retired, I tend to go out in the middle of the day, which is definitely old people and retiree time. I was vaguely aware that there may be people of other ages that had to be elsewhere during the day but did not realize how many until this pandemic resulted in everybody being at home all the time. Now when I go out during the day for a walk, even with physical distancing, I see young parents with infants in strollers, toddlers, children playing outside, teenagers listening to their phones, joggers, and young and middle-aged people and realize that I live in a much more age-diverse group than I had thought.