Back after a week off – exam, and an infection. I think it may be hard to (re)establish momentum.
I don’t know how far the class got on regression in my absence Tuesday. I think I will assume “nowhere”, and start from scratch. But not with the Tower of Pisa. I think I will recreate the crime up / fear down charts in the book, and interpret the slopes in words. And try to have this done in groups.
Spoke with Maura just now about having to save myself from becoming too radical. I think the course (the book) could do without linear equations. In the exam, one thoughtful student solved the CFL payback problem correctly even though she couldn’t write down the equations. She wrote that she knew her equations were suspect, so didn’t want to trust them. She decided just to “figure it out” – which is, after all, the whole point of the exercise.
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I think that was a good class, on regression. I worked through crime rises fear falls as planned, imagining the data as embedded in a sociology paper. I began with a digression on downloading data from the web – best if the web site offers an excel option, else cut and paste into word, insert commas and save as .csv. It was an adventure figuring out how to do that on a mac.
Then we drew the first scatter plot from the book and discussed what the trend might “mean”. Then we eyeballed a trend line and drew one in Excel. We interepreted the slope as a rate. (In general I think the class went well – the students absorbed the important ideas. But I’m not sure of they grasped this particular idea.)
We had time to draw the graph interchanging dependent and independent variables, and make up fake causation in that case too. No time to use time as an independent variable.
One student asked what R^2 was for. Good question. It’s a measure of confidence. I did an artificial linear example in Excel to show that then R^2 would be 1 and Excel’s equation would match the one we used to compute the values. I think that was informative – I know there’s an exercise like that in the book. Maybe it should be even more prominent. The student who asked the original question immediately read R^2 = 1 as “100% sure”!
We finished with the xkcd cartoon that heads the chapter. Half the class even got it.
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