Think Stats

Chapter 3. Cumulative Distribution Functions

The Class Size Paradox

At many American colleges and universities, the student-to-faculty ratio is about 10:1. But students are often surprised to discover that their average class size is bigger than 10. There are two reasons for the discrepancy:

• Students typically take 4–5 classes per semester, but professors often teach 1 or 2.

• The number of students who enjoy a small class is small, but the number of students in a large class is (ahem!) large.

The first effect is obvious (at least once it is pointed out); the second is more subtle. So let’s look at an example. Suppose that a college offers 65 classes in a given semester, with the following distribution of sizes:

 size      count
 5–9           8
10–14          8
15–19         14
20–24          4
25–29          6
30–34         12
35–39          8
40–44          3
45–49          2

If you ask the Dean for the average class size, he would construct a PMF, compute the mean, and report that the average class size is 24.

But if you survey a group of students, ask them how many students are in their classes, and compute the mean, you would think that the average class size was higher.

Example 3-1. 

Build a PMF of these data and compute the mean as perceived by the Dean. Since the data have been grouped in bins, you can use the mid-point of each bin.

Now find the distribution of class sizes as perceived by students and compute its mean.

Suppose you want to find the distribution of class sizes at a college, but you can’t get reliable data from the Dean. An alternative is to choose a random ...