Social Science and Statistics in the PA Voter ID Decision

Courtesy of Pennsylvania DMV

I’m often asked, particularly by junior faculty, about how social scientists get involved in litigation.  A good way to learn how statistical reasoning gets used in legal cases is to review cases.  The “All About Redistricting” website maintained by Justin Levitt at Loyola Law School, and the “litigation page” at the Moritz College of Law at The Ohio State University, for example, are treasure troves of case materials.

The recent Pennsylvania trial court ruling striking down the state’s voter ID law is only the most recent instance where a court relied heavily on evidence produced by social scientists and statisticians.  (All the page numbers referenced below refer to this decision.)

The full set of documents pertaining to the case can be found at the Moritz site (unfortunately many of the documents are low quality scans and can’t be searched).

The Findings of Fact are a good place to start (pg. 54 of the decision) because they summarize the source of the evidence submitted to the court and can often used to quickly identify expert witnesses.

The Moritz site is pretty comprehensive for this case, including most of the expert witness reports.  The social scientists used in the case were:

  • Dr. Bernard Siskin did most of the statistical analysis for the plaintiffs. Siskin was a long time professor of statistics at Temple and now appears to be a full time expert witness, mainly working on employment discrimination.  
  • Dr. William Wecker is another statistician who works exclusively as an expert witness; previously he was a tenured professor of business at the University of Chicago.  Unfortunately, I could not find Wecker’s report on the website.
  • Dr. David Marker, a senior statistician at Westat, a statistics and data collection firm that I believe started in the Research Triangle area of North Carolina, but which has grown worldwide.   Marker was hired solely to evaluate the survey methodology used by Dr. Matthew Barreto in research that has often been cited as demonstrating racial and ethnic disparities in access to voter ID in Pennsylvania.
  • Dr. Lorraine Minnite is an associate professor at Rutgers-Camden and a well-known expert on vote fraud.  Minnite was brought in by the plaintiffs to examine the content of the legislative debates regarding the need for voter ID and the prevalence (or not) of voter fraud in Pennsylvania.  Her report starts at page 20 here and is an entertaining read for anyone interested in legislative intent.  The court determined, for instance, that there appeared to be a “legislative disconnect from reality” (pg. 41 of the decision), and Minnite shows that, whatever the merits of voter ID, the speculations of legislators often outstripped reality.
  • Dr. Diana Mutz is a professor of political science at University of Pennsylvania best known for her work on political communication, political psychology, and public opinion.  I was surprised to find Mutz’s name among the witnesses; her testimony was used by the plaintiffs to try to show that the state had insufficiently advertised how citizens could obtain an ID.

Aspiring expert witnesses can learn at least two lessons from this case.

Learn the tools: Siskin’s report is a virtual manual for matching complex databases to estimate racial and ethnic disparities.  A key piece of evidence was Siskin’s estimate of the number of PA citizens who did not have valid photo IDs.  The work involved fuzzy set matching of Penn DOT and voter registration databases (pg. 62 “Scope of Need”; pg. 17 of the expert witness report); he used “BISG” methodology to estimate racial disparities even though his data sources did not contain racial or ethnic identifiers (pg. 20 of the report); and relied on “Open Street maps” data to estimate drive times for residents without IDs to the closest drivers license office (pg 27 of the report).

You don’t necessarily need to use the most advanced technology (Siskin uses SPSS for all of his statistical estimation), but your methodology must be scientifically sound.

Honor scientific standards of evidence:  Wecker was hired by the defendants solely to, in the words of the Court, “refute Dr. Siskin’s work.”  The court’s treatment of Wecker’s evidence is illustrative of what happens if your evidence can be criticized for not following conventional scientific practice.  The court refers to the testimony as “flawed and assumption laden”.

Compare this to the court’s treatment of Marker, and by implication Barreto, both of whom followed valid scientific standards.

A nice introduction to expert witness work was penned by Dick Engstrom and Mike McDonald in 2011.  It’s a nice exercise to read Engstrom and McDonald’s useful essay and then review expert witness reports in this and other cases.