Fish Story

This winter I volunteered with our local Watershed Council to analyze data on growth rates of cutthroat trout. Estimating growth of anything requires at least two measurements, one later than the other, with growth rate expressed as increase over time. The equivalent of the student ID number for fish is the tracking tag, a magnetically encoded implant something like a pet ID. Over a period of years, field volunteers had captured and measured the same tagged fish several times along multiple streams.

For an evaluator from education, where growth measures are all the rage, how hard could this be? Harder than you might think. For instance, the data showed some trout as being smaller on recapture than on the day they were tagged. If these were kids, I might attribute the decline to curriculum, instruction, motivation, or test validity—but I would keep the data. No, the Council told me. Fish don’t shrink. Those are measurement errors attributable trying to hold a ruler up to a slimy noncompliant subject. Omit them.

Then there were outliers, fish that seemed to grow several millimeters a day. Surely I could omit those data? Not necessarily, the Council said. You have to understand how trout live. Some spend their lives in small streams and remain small. Others migrate to and from larger rivers and grow big. I had to split the numbers out by where the fish were trapped.

By then it was evening and the news came on. I learned that employment is not keeping pace with population growth, but unemployment is the lowest it has been in decades. The national budget deficit is huge, but the rate of increase has shrunk dramatically. Health care costs are increasing, but not as much as they used to.

Accepting either half of one of these statements by itself makes a big difference in what you believe and what actions you support. The lesson for education is that we need a people who both know how to count and who know what counts. The only resolution of facts and alternative facts is a willingness to go back where numbers come from and try to grasp the slippery fish.