When policymakers don’t understand basic statistics
Imagine, if you will, that the Stuarts have two children, Kylie and Frederick. Kylie was born in 1997 and Frederick was born in 2001, both in Toledo, Ohio. The Stuarts measure Kylie’s height when she reaches the spring of fourth grade, in 2007, and learn that she is 52” tall, or 4’4”.
Shortly thereafter, the family moves from Toledo to Kansas City, Mo., and Mrs. Stuart decides to change the family’s diet, reducing the red meat content and adding more whole grains and fresh fruit. When Frederick is in the spring of his fourth-grade year, in 2011, the Stuarts measure his height. They find that Frederick is 54” tall, or 4’6”—two inches taller than Kylie was at the same age.
Mrs. Stuart is ecstatic. “The family’s height has grown over time!” she exclaims. “Our fourth-grader’s height in 2007 was 52 inches, and our fourth-grader in 2011 was 54 inches tall. That’s growth over time!” Giddy, she says, “And I’m sure that the growth is because of the change in diet. It really works! If other families did this, their kids would grow just as much as ours did from 2007 to 2011!”
Mrs. Stuart may not know it, but she has a future in politics. The mangled logic on which she relies is no different than what emerged last week from the mouths of well-known policymakers upon the release of the 2013 scores on the National Assessment of Educational Progress (NAEP).
Every two years, the federal government assesses a sample of fourth- and eighth-grade children in reading and mathematics in all 50 states and the District of Columbia, as well as students in a sample of urban school districts. When the scores are released, policymakers and pundits scramble to
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