Sixty symbols

A family friend put me on to the great Sixty Symbols (cheers, Cam)—a collection of videos featuring academics at the University of Nottingham. Each one is focused on a symbol with some important meaning in physics: γ links to a five-minute explanation of the Lorenz factor and time dilation; ψ to ten minutes on the wave function. Here’s Laurence Eaves and Mark Fromhold on chaos and the butterfly effect.

When you’re done, there’s also a chemistry sister site, The Periodic Table of Videos.

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Bad science

Ben Goldacre has a new TED talk. It’s very good, and worth watching even if you’re already familiar with his work exposing bad, and often dangerous, science.

In fact, 76% of all of the trials that were done on [reboxetine] were withheld from doctors and patients. Now if you think about it, if I toss a coin a hundred times, and I’m allowed to withhold from you the answers half the time, then I can convince you that I have a coin with two heads.

His blog, Bad Science, is usually great read (as is his secondary blog).

Correlating angles (circular variables)

I’ve been trying to find a sensible correlation coefficient to assess the reliability of the cylindrical axis on an optometric prescription. Glasses can have spherical correction, which has equal refractive power in all axes; but they can also have cylindrical correction, which has power in a specific axis to correct for astigmatism. So a prescription might include one or more cylindrical lenses, which need to be set at a specific angle. In our PERGENIC work, we corrected participants’ vision when necessary, but we wanted to be sure that we were doing it reliably. We asked 10% of participants to return for a second session, and when they came back we performed a second refraction without referring to the results of the first one—that way, we could compare the two sets of data to make sure they were consistent.

So far, so good. But you run in to a problem if you try to calculate, say, a Pearson product-moment coefficient.1 Here’s the problem: an angle is modular. For a cylinder angle, 0° is exactly the same as 180°. So if the first time I prescribe correction at 179°, and the second time I prescribe correction at 1°, there’s only a 2° difference. But it looks like a 178° difference. Which is not very good.

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