Chance News 59

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I wish I could say, well, I told people correlation doesn't equal causation back in 1989, so I don't have to say it again.

Joe Palca, NPR science correspondent, lamenting the lay public's continual lack of understanding of how science progresses; this appeared in an article by Julia Galef entitled "Uncertainty in Science, It's a Feature, Not a Bug," The Humanist, January-February 2010.

Submitted by Paul Alper


Calculating high school dropout rates

KC School District's dropout rate doesn't add up. Michael McShane, The Kansas City Star.

The Kansas City, Missouri school district had some amazing statistics to brag about.

The Kansas City School District recently announced a dropout rate of 5.9 percent. Compared with the dropout rate of 41.2 percent reported a year ago, it appeared as if the district was moving by leaps and bounds in the right direction to correct the problem.

These results, however, appear to be incorrect.

The Missouri Department of Education says when the Kansas City School District’s Class of 2009 started eighth grade in the fall of 2004 it had 2,629 members. When that class graduated this spring, 1,032 students earned diplomas. It doesn’t take a degree in mathematics to recognize that does not add up.

The calculation of a dropout rate is not too difficult.

It is a simple mathematical formula; take the total number of students who graduate and divide it by how many students started in eighth grade. If necessary, adjust that number for demographic movement trends and with a No. 2 pencil and a scientific calculator, anyone at home can estimate the graduation rate.

Here are the numbers you need for the calculation.

Let’s calculate it together. When those 2,629 eighth-graders were enrolled in the district, the total enrollment for the district was 26,968 students. When 1,032 members of that cohort earned diplomas there were 22,479 total students enrolled in the district.

If you don't account for migration, the graduation rate is 1032 / 2629 = 39%. Here's how to account for migration.

In that same period, the overall district enrollment declined by 16.65 percent, so it’s fair to reduce the number of eighth-graders to reflect that, which we can do by multiplying by 0.8335. After those calculations, the adjusted graduation rate of the district is really 47 percent.

Part, but not all of the discrepancy, can be accounted for by a change in time frame. The 5.9% represents an annual drop-out rate, not a rate across four years, a practice that Dr. McShane derides.

The district’s using that number as its dropout rate is the equivalent of your credit card company telling you the monthly rather than the yearly interest rate. It may make you feel better, but you’re still going to pay big.


1. How would you convert a yearly dropout rate to a four year dropout rate?

2. The adjustment for migration makes some assumptions. What are those assumptions? Are they reasonable?

3. Would it make sense to compute confidence limits for the dropout rate?

Submitted by Steve Simon

Item 2