Wednesday, August 3, 2011

Causal inference in the Dutch Parliament.

A friend recently sent me the video below. Its a discussion taking place in the Dutch Parliament about what works better to avoid backsliding: community service or a prison sentence. One MP notes that recent reports show that community service works better - compared to somebody with community service, a person with a prison sentence is twice as likely to backslide. The MP at the stand (Lilian Helder from the PVV) doesn't belief this. She keeps on saying "You can't compare person A with person B. They are different."

This video received a lot of attention. Up to now, it has already received more than 300,000 views, 2,000 comments and almost a 1,000 "likes". Moreover, the reactions to this video are all of a particular kind, nicely reflected in the title "PVV lady doesn't understand statistics".

Lilian Helder is an MP from the Partij Voor de Vrijheid (PVV) - indeed, the infamous Dutch right-wing party of Geert Wilders. The people that read this blog know that I'm against almost all that this party stands for: anti-Islam, anti-immigration, anti-development aid, anti-Europe, etc. As a result, any possibility to take a shot at the PVV I take - even cheap ones. However, Lilian Helder - probably without knowing it herself - touches on a fundamental issue in statistics.

Person A is indeed different than person B. In order to make a proper causal claim one would have to compare the occurence of backsliding by person A that has community service with the occurance of backsliding of exactly that same person A that has a prison sentence. Of course this is not possible - we only live one life. This problem is so important in statistics that it is called "the fundamental problem of causal inference".

Comparing person A with a community service and person B with a prison sentence does indeed not make sense because they are different people. Any difference in occurence of backsliding between these two might very well be because of individual characteristics. Maybe person A is well-educated and quickly finds a job, while person B is not and thus might backslide. It is true that one can use statistical regression techniques to control for these factors. However, not all variables can be measured (psychological variables for example), and one can never be sure that all the necessary variables are included in the regression. These are very important problems in causal inference.

In order to make a correct causal claim one has to make sure to find a correct comparison. One way to do this is by a so-called Randomized Control Trail (RCT). Under this technique instead of individuals one compares groups. In our case above it would look as follows (I'll keep it brief). Let's say we have 10,000 people that have to be punished. We then randomly select 5,000 people for community service and 5,000 people for a prison sentence. Because these two groups have been selected randomly, the groups will have the same characteristics. For example the number of people that backslide in each of these two groups (community service vs prison sentence) can then be compared. While this technique has been used in bio-medical sciences for decades they have only recently been introduced in the social sciences. For a more complete discussion (with development aid as an example) please see here (in English) or here (in Dutch).

I haven't read the studies that the MPs refer to but it is very well possible that they did not make use of an RCT or a related strategy to make these causal claims. I have difficulties writing this, but Lilian Helder's remarks might not have been that dumb.


  1. I hate to do this, but I don't think the RCT is new to the social sciences. Randomization has been applied to the social sciences for decades. Examples can be found in psychology and marketing where respondents are randomly assigned to experimental conditions to determine the effects of all kinds of things.

    But it's great that other social sciences are starting to use the method too :)

  2. The previous poster is correct. The RCT has been a staple of psychological research (including criminology) for many decades now, even to the point that some scientists are criticizing the tendency to almost completely rely on this method. I can assure you that randomization is one of the very first principles taught to psychology students and that it is gathering more attention in other social sciences as well (rightfully so, as you point you!).

    Sometimes it is of course not possible to use a RCT, but in those cases psychologists are expected to find other ways to make the two groups as comparable as possible (for instance, by carefully matching the two groups on as many characteristics as possible).