One of the first “reality checks” that you have to accept when starting a self-tracking project is the existence of measurement errors. No matter how technologically advanced are the measurement and recording tools you are using, these instruments will never be able to capture the “true value” of the object or trait that you attempt to measure. For that, you can blame two types of errors: “systematic” and “random”.
NOTE: this post was originally published on this blog in October 2012. I am unarchiving and reposting it as a part of the Measured Me 2.0 initiative. You may see comments that date back to the time of the original posts.
Take, for instance, body weight. If you weigh yourself several times throughout the day and then chart the results, you will notice considerable fluctuation. Your weight will naturally go up right after breakfast and even more after lunch, and your gym scales will most likely show different results than your scales at home. These differences are caused by so-called “systematic” error. The systematic error is an error that occurs due to the misuse of instruments and changes in measurement procedure. In the above-mentioned example, you weighed yourself in different time of the day, using different scales, etc. Every time you introduce a change in the measurement procedure, you affect the results.
Luckily, systematic errors can almost often be eliminated or minimized by means of standardization of the measurement process. In other words, you fight systematic error by introducing the system: find the optimal conditions under which the measurement is assumed to be most accurate, and from now on stick to the same routine. In the case of body weight, my solution is to weigh myself completely naked every morning right after the shower and morning toilette routine (i.e., after emptying the bladder and bowels). If for some reason, my morning toilet was not successful, I prefer not to record my weight on that day.
Tip: I am currently using this awesome USB rechargeable digital scale to track my weight, which I found to be a more efficient and affordable alternative to Withings scale.
Another type of error that will always follow you on your self-tracking endeavors is the “random” error. This is a “noise” that occurs due to uncontrollable factors and nature of the object that you are trying to measure. I would say that the more intangible is the object of measurement, the higher is random error. For instance, latent characteristics such as emotions, mood and other psychological traits are the most difficult to capture. In this case, taking multiple measurements (or better, multiple instruments) and averaging out results could help to minimize the error. For example, I take “snapshots” of my current physical, mental and emotional state multiple times throughout the day. The daily average is the value I would use if I wanted to analyze how weather or some other factors affect my mood, for instance.
Finally, remember: in self-tracking projects, it is almost always relative changes and comparison that matter the most. In other words, it is not the absolute value of whatever you measure that matters, but the relative change in its values across time periods or treatments. If you keep measurement routine the same, then you will still be able to capture the change. So even if your bathroom scale constantly underestimates your weight by 1 lb you will still be able to see your progress, as long as you continue using the same scale.