Accuracy and Precision

I climbed on our new bathroom scale for the first time last night. 188.8 lbs, it reported. I stepped off, waited for it to reset, and stepped on again. 188.8 pounds. That made me very happy. What are the chances I actually weighed 188.8 pounds? Pretty remote, I suspect. But I don’t need the scale to be accurate, I need it to be precise.

Rewind to the old bathroom scale. When I resolved to monitor my weight with actual numbers (which can be a trap), the light of my life obliged by bringing an old-school spring-driven scale into the bathroom. Cost: eight bucks. The thing was, shifting my weight or moving a foot would change the readout. If you weren’t concerned about exact numbers, the scale was plenty adequate for measuring a trend.

Once I set goals with exact weights involved, however, the cheapo scale became a source of frustration. Leaning forward to read the dial better changed the reading. Am I allowed to drink beer today or not? Answer unclear, ask again later.

My sweetie set out once again to find a scale that could answer that question. At CVS she found a thirty-dollar scale programmed to give you terribly ill-informed body-mass advice, a twenty-dollar one that… I don’t remember what its deal was, and a simple, ten-buck CVS-branded scale that rests in our bathroom now, easy-to-read and frightfully consistent. And precise. This morning the scale told me with giant LCDs that I weighed 188.4 pounds. I stepped off, waited, then stepped on again. 188.4. I smiled. That’s 1.6 pounds below the beer threshold for this month, and roughly ten pounds lighter than I was three weeks ago. Or five pounds. Or fifteen. Hard to say.

But next month, even if I remain skeptical of the number on the scale, I will be confident of the difference. And the difference is what this whole project is about.

2 thoughts on “Accuracy and Precision

  1. I learned the difference between accuracy and precision when I was a “weight lifter” at the Lab, checking each day the balances that measured some pretty serious “stuff.” (The weights were 2 kilos down to a few grams)

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