Thu Mar 14, 2013

U.S. News's Small N Problem

While we’re running our Crowdsourced Sociology Rankings, people have been looking a little more closely at the U.S. News and World Report rankings. Over at Scatterplot, Neal Caren points out that U.S. News’s methods page has some details on the survey sample size and response rates. They’re bad:

Surveys were conducted in fall 2012 by Ipsos Public Affairs … Questionnaires were sent to department heads and directors of graduate studies (or, alternatively, a senior faculty member who teaches graduate students) at schools that had granted a total of five or more doctorates in each discipline during the five-year period from 2005 through 2009, as indicated by the 2010 “Survey of Earned Doctorates.” … The surveys asked about Ph.D. programs in criminology (response rate: 90 percent), economics (25 percent), English (21 percent), history (19 percent), political science (30 percent), psychology (16 percent), and sociology (31 percent). … The number of schools surveyed in fall 2012 were: economics—132, English—156, history—151, political science—119, psychology—246, and sociology—117. In fall 2008, 36 schools were surveyed for criminology.

So, following Neal, this tells us the Sociology rankings are based on a survey of 117 Heads and Directors with a response rate of 31 percent, which is thirty six people in total. For Economics you have 33 people, for History 29 people, for Political Science 36 people, for Psychology 40 people, and for English 33 people. The methods page also notes that they calculate the scores using a trimmed mean, so they throw out two observations each time (the highest and the lowest). The upshot is that the average score of a department is likely to have rather wide confidence intervals.

Update: These numbers are too low. Read on.

I guess it’s possible that U.S. News might mean that the effective N of, e.g., the Sociology survey is 117, and that’s the result of a larger initial survey which yielded a 31 percent response rate. On that interpretation they initially contacted 378 departments (or thereabouts). That would be a non-standard way of describing what you did. Normally, if you give a raw number for the sample size and tell us the response rate, the raw number is the N you began with, not the N you ended up with. More importantly, there aren’t 378 Ph.D granting Sociology departments in the U.S.—a quick check of the Survey of Earned Doctorates suggests that there were 167 in 2010. That suggests that 117 is about right for the number who had awarded five or more in the past five years, and that this is the total initial N. Same goes for Economics, which has 179 Ph.D programs in the 2010 SED.

Then again, the wording in the methods can also be read as saying every department received two surveys (“Questionnaires were sent to department heads and directors of graduate studies … at schools that had granted a total of five or more doctorates … during the five-year period from 2005 through 2009”). Looking more closely at the available SED data for 2006 to 2010 (one year off the USNWR dates, unfortunately), I found that 115 Sociology Departments met the stated criteria of having awarded five our more doctorates in the previous five years. If both the Dept Head and DGS in all those departments got a survey, this makes for an initial maximum N of 230. But that is still quite far from the 378 or so needed, if 117 is supposed to mean the 31 percent who responded rather than the total number initially surveyed.

So it seems like the most plausible interpretation is that for Sociology the number of schools surveyed is in fact 117, that every school received two copies of the questionnaire (one to the Head, one to the DGS or equivalent), but that the 31 percent response rate means “percent of schools from which at least one response was received”, and so the total N surveys for Sociology is somewhere between 36 and 72 people, with a similar range of between 30 and 80 for the other departments.