We improvise, guess, forget our results or change the conditions without even noticing the results. Errors are possible in self-tracking and self-experiment, of course. It is easy to mistake a transient effect for a permanent one, or miss some hidden factor that is influencing your data and confounding your conclusions. But once you start gathering data, recording the dates, toggling the conditions back and forth while keeping careful records of the outcome, you gain a tremendous advantage over the normal human practice of making no valid effort whatsoever.