Last class before break. I’d hoped to do a clean lecture/tutorial on finding mode, median and mean from histograms. The class went well as far as it went, but now we have a week off. Restarting will be difficult.
I worked the US Household Income Distribution exercise, starting from this image from wikipedia:
It took nearly 2/3 of the class just to read the graphic and understand what it means, and think about mode, median and mean without trying to do the exact calculations.
The mode was easy – highest bar. Though it’s hard for them to remember that the mode is the income at thehightest bar, not the height of the bar.
For the median I asked the class to imagine all 100 million households lined up across the front of the room, to illustrate that the median income would belong to the family in the middle.
Then with all the families lined up, we could imagine computing the mean by adding up the 100 million incomes and dividing by 100 million. But if the families left and left behind only the summarized data in the graphic we would have to … I wrote out the equation for the weighted average.
Then it was time to implement the calculations in Excel. Unfortunately, all the links to the spreadsheet containing the data were broken. The time I took fumbling around for it and then entering the numbers by hand in a fresh spreadsheet meant that I didn’t have time to use the spreadsheet as much as I’d wanted to. We did draw the histogram, talked briefly about fake 3d, then went on to the mean.
The most interesting new software feature was Excel’s decision to update row and column references on cut and paste. Most of the class got it, and felt the wow! moment. But along the way the physical cutting and pasting was hard. Some mice lacked right buttons.
At the very end I was able to show how assumptions about the missing three percent of households could affect the mean, asking “what-f” questions and letting Excel update all the computations. But it was too near the end of the class, too near break. I think I may want to do this whole class over again a week from Tuesday.
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