Moving Average and Weight Loss (Where Geekness and Fitness Come Together!)

I glanced back at my past several post and realized I mention the phrase "moving average" in almost all of them. So what the heck am I talking about? When you are monitoring your weight, there will be day to day fluctuations. Depending on your hydration levels, changes over the course of the month (ladies, I think we all know what I'm talking about here) sodium intake and the -ahem- progress of food through your system, it's easy to fluctuate 3, 5 or more pounds over the course of a day, or from day to day.

K-Lo, who is on the 5 Pound 5 Week Challenge with me, lost 5 pounds in 3 hours one morning. She had a particularly salty meal the previous night, woke up bloated and retaining water, and as her body rebalanced her hydration levels, normalized to a more accurate weight.

Obviously, eating clean and working out consistently and seeing a 3 pound weight gain on the scale can be really discouraging, which is why a lot of weight loss programs advocate only weighing yourself weekly.

But I think the ritual of the morning (or evening or whenever, as long as you are consistent) weigh-in helps to focus one's attention and set a tone of healthy weight management for that day. And, as someone who has been quite overweight, I prefer to catch upward trends sooner and nip them in the bud with small behavior changes, rather than waiting three or four weeks to see a trend emerge and then having to deal with 5 or 7 pounds to lose. (Think you can't gain 5 or 7 pounds in a month? Au contraire, mon fraire. Two words: Christmas Cookies.) In any event, my morning weigh-in is like brushing my teeth: just one of those things I do everyday.

So if you are going to weigh yourself daily, how do you avoid making yourself crazy when confronted with sudden gains and losses? You factor in the moving average of your weight, which shows the trend of weight loss and damps down the day-to-day fluctuations. Investors use moving averages (simple and more complex versions) to get a better idea of the trend of stock prices without getting caught up in day-to-day market fluctuations. It's a pretty neat and very simple tool for analysis of something over time.

A simple moving average is calculated by tracking your weight over a given number of days, adding up the total pounds from that time period, then dividing the sum by the number of days. To get a four-day simple moving average of your weight, you would add together your weight over the last four days, and then divide that number by four. Perhaps some charts would make this clearer:

This is the chart of my weight since 4/1/09, when I began the 5 Pounds, 5 Weeks Challenge. Note the big uptick on 4/8 and the big downtick on 4/10. Both of these were times that could have been cause for a lot more emotionality than they really deserved. It would have been easy to feel a bit despondent on 4/8, like all my hard work was ruined and I was back up to my 4/2 weight from a week earlier. But that would have been a false interpretation of the trend, as the moving average will soon demonstrate. Likewise, while it felt good emotionally to hit 151 pounds on 4/10, I knew immediately I was looking at a false low and I would see some rebounding the next day or two. If I hadn't realized that, the 2 pound weight gain the next day could have really taken the wind out of my weight loss sails.

So here's that same data, analyzed with a four-day simple moving average. The upticks and downticks are damped out, and a much more realistic picture of my weight loss progress becomes clear. Instead of a seemingly random collection of ups and downs, a slow, gradual decrease comes into view. This is a believable picture, showing about a 3 pound weight loss over 9 days, and is far more likely to represent actual fat loss, which is of course the goal, instead of changes in my hydration.


Now here's those two charts viewed together. Which line do you think gives you a clearer feedback picture? The Moving Average, natch. You don't have to actually create an Excel spreadsheet to figure all this out and calculate your own moving average (but it's really easy and if you do you get bonus geek points!) but if you are tracking your weight, it's a good idea to at least keep the moving average idea in the back of your head before you jump to conclusions about a "ruined diet" or "breaking through a plateau." Sometimes it's enough to glance back over the past several days of your weight log: are the past few days when taken together generally lower than those same days last week? Then you are probably on track.

There's quite a few tools to help with this concept online. There's the Google 15, an awesome little moving average calculator widget that you add to your Google Homepage. Old School .xls speadsheets are available with the (free! free! free!) Hacker's Diet, the most effective and honest (and did I mention free?) weight loss book around, assuming you can handle the geek-truth contained in it's pages. And if reading this has just made you tingly to know more about statistics, Wikipedia's Moving Average page goes into far more detail than I ever will.