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Jevin West was away today so in lecture I was able to sneak in one my favorite topics, observation selection effects.
Let's start a little puzzle.
In Portugal, 60% of families with kids have only one child. But 60% of kids have a sibling.
How can this be?
Let's start a little puzzle.
In Portugal, 60% of families with kids have only one child. But 60% of kids have a sibling.
How can this be?
People are all over this one! And some are out ahead of me (looking at you, @TimScharks). We'll get there, I promise!
There are fewer big families, but the ones there are account for lots of kids.
If you sampled 20 families in Portugal, you'd see something like this.
There are fewer big families, but the ones there are account for lots of kids.
If you sampled 20 families in Portugal, you'd see something like this.
Now let's think about class sizes.
Universities boast about their small class sizes, and class sizes play heavily into the all-important US News and World Report college rankings.
For example, @UW has an average class size of 28.
Pretty impressive for a huge state flagship.
Universities boast about their small class sizes, and class sizes play heavily into the all-important US News and World Report college rankings.
For example, @UW has an average class size of 28.
Pretty impressive for a huge state flagship.
But you ask students, their experiences tends not to reflect these numbers.
Students typically report that almost all of their classes were bigger than the listed average class size.
How can that be? Are universities lying about average class size?
Students typically report that almost all of their classes were bigger than the listed average class size.
How can that be? Are universities lying about average class size?
Not at all.
But average class size isn't the most relevant metric for *students*.
Classes are like families in Portugal. Most of them are small, but because they are small, a minority of individuals are in them. Most students are in the comparably few huge classes.
But average class size isn't the most relevant metric for *students*.
Classes are like families in Portugal. Most of them are small, but because they are small, a minority of individuals are in them. Most students are in the comparably few huge classes.
It's surprisingly hard to get data on class sizes, but Marquette University provides binned data. If we use bin averages, we get the table below. Average class size is 26. Average *experienced* class size, the average size experienced by a student, is very nearly twice that, 51.
This leads to some cool results. If you stood on campus and asked passers-by "How big is your next class", the professors' answers would average 26 (there's one prof per class) but the students' would average 51 (bigger classes have more students in them).
To be continued....
