Saturday, March 27, 2010


You may have heard of a category 5 hurricane... There's also a scale (category 1, 2, 3...) scale for tornadoes called the Fujita scale. F5 is the strongest tornado possible (although there's a mythical F6 "inconceivable tornado" category that starts above ~512 km/h (318 mph) but has never been used).

From what I've read, Ted Fujita, who developed the scale, was a neat guy. I could imagine him being mentioned in a Bill Bryson book, for example. I once read that on his days off he loved to do little experiments like measure and plot how his weight changed throughout the day. I saw one of these charts and it was annotated with bits like "mowed lawn", "afternoon snack". Years ago, when googling, I also saw a daily chart of weight on Robert Balling's webpage (he was a climatologist in Arizona) and thought it was fascinating.

Now, I also vaguely remember that my dad did something similar, that he'd weigh himself daily and write it up on a calendar. He had had a series of heart attacks so keeping his weight down was a serious issue. I've since read that they discourage you from weighing yourself every day, just so it doesn't become an obsession, or put you in a foul mood when there's some random fluctuation.

But I say "bah!" to all that. Why not collect the data and know and understand? So this below is my weight, measured first thing in the morning, since moving to Australia. On the left is pounds, on the right kg, with a 10 day moving average.

A wee bit of time series analysis showed that the daily standard deviation is somewhere between 1-1.5 pounds. Even more interesting, there's a negative autocorrelation, so that any rise or fall in weight sticks around on average for 2-3 days but then 10-13 days later rebounds in the opposite direction. The most I've ever gained in a day was 4.2 lbs at the end of October, followed by a record loss of 3.8 lbs the next day, so likely just water weight. It was no doubt due to beer. I can starve myself all day, but have one beer and I inevitably skyrocket the next day.

As for why it goes up and down over the long term, hard to say for sure. Looking back on the calendar, the rise in July might have been during a very busy and stressful time at work and when my mom visited. The slump in October happened when Kitty went to southeast Asia. I reckon there were fewer collateral calories during that period, as evidenced by a total absence of chocolate bar wrappers in the bed. Then new years' rise was because of a 1 week non-stop meal. More recently I've tried to cut ramen soup and crackers out of my diet.

Knowing all this, it's hard for me to say what the best weighing strategy is. I'd probably say every two weeks is fine, keeping in mind that changes of less than 3% of your body weight are within the limits of noise.

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