Plan: Continue with taxes, working toward linear equations in Excel. Perhaps start or end with electricity bills. One possibility: social security taxes, which have two brackets, one with rate (about) 12.5% (half paid by employer), then zero! Show how this plays out when computing effective tax rates. Look at capital gains tax (lower rate). Perhaps study the spreadsheets from the book, which have some pretty subtle Excel constructions. That will show them some of what’s possible in Excel – from a read rather than write perspective. I don’t expect them to be able to build applications like those.
Try not to move too fast – maybe an electricity bill interlude first. That’s a simple linear model, with which to introduce scatter plots (not line charts!) and the Excel $ref feature.
It’s hard to teach in the basement of the library when it’s 80+ degrees and sunny outside on March 22.
I offered three choices:
- really work on histograms, mean, median and mode
- play on line with our imagined taxpayer’s return
- start at the beginning of the chapter with electricity bills, leading to linear equations. (The tax material is the end of that chapter.)
Much to my surprise, almost all wanted to do the first of these. I said that anyone who thought they knew the material was free to leave to sunbathe; one student took me up on the offer. For the rest, I did the home page usability exercise from the homework, first on the board and then in Excel. It went really well. I almost want to say surprisingly well.
In particular, we went slowly through the copy-paste-watch-Excel-change-cell-references. That was very cool – particularly at the end when we decided to recompute the mean using 0, 10, 20, … rather than 5, 15, 25, … for the average score in each bucket. That was a single change in one cell – the Excel did the recalculations automatically.
I’d intended to get to =SUMPRODUCT() to show how to compute weighted averages in fewer steps, but ran out of time.
Along the way I discovered several mistakes in the posted solution, which I will fix tonight or tomorrow.
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