I’ve diligently recorded my weight every several days for some years now. The short-term variation always astounds me—going up and down by a number of pounds from one week to the next. However, on average month-by-month my weight remains surprisingly stable. Unfortunately the trend over years is very slightly, but significantly, upwards. Thus my battle against the bulge continues.
A few weeks ago I stepped up for a weighing and received a pleasant surprise: It seemed that I’d dropped 10 pounds during a five-day business trip. Although deep down I knew this could not be, I indulged myself for the day with the thought that something magical had whisked away this weight. Then after getting home from work I got back down to earth by discovering that the base of the scale had got off kilter. I mentioned this to my wife and daughter. It was funny seeing them being so crestfallen—they also were hanging on to the belief in a mysterious, but real reduction.
I suppose all this supports the use of control charting* for filtering out common-cause variability (not worth reacting to) from statistically-significant process upsets (special causes that merit attention). At least that’s what my logical side says. On the other hand, it was fun to believe for some hours in supernatural forces. Ignorance can be bliss!
*(See this detailing posted by the George Mason University College of Health and Human Services.)