In praise of experiments

Today, the excellent Neuroskeptic writes about a new study investigating which US states are most suicidal. The interesting twist was the form of the data: Google searches. It's an interesting study and an interesting use of Google searches, but what struck me was Neuroskeptic's closing thoughts.

Over the past couple of years there's been a flurry of studies based on analyzing Google and Twitter trends. What's interesting to me is that we're really in the early days of this, when you think about likely future technologies. What will happen when everyone's wearing a computer 24/7 that records their every word and move, and even what they see? 

Eventually, psychology and sociology might evolve (or degenerate) into no more than the analysis of such data...

It's always dangerous to predict the future, but here's my prediction: Not a chance. It gets down to a distinction between observational studies and experiments. Observational studies (where you record what happens in the course of normal events) are useful, particularly when you care about questions like what is the state of the world? They are much less useful when you want to know why is the world the way it is?

There are a couple reasons.

Reason #1: The correlation fallacy

First, observational studies are really about studying correlations. To have much power to analyze interesting correlations, you need a lot of data. This is what makes Google and Twitter powerful: they provide a lot of data. But correlation, famously, doesn't always tell you much about causation.

For instance, it is now well-known that you can use the number of pirates active in the world's oceans and seas to reasonably predict average global temperature (there's a strong correlation):

I did not know until recently that Google search data has now definitively shown a correlation between the amount of movie piracy and global warming as well:

In the case of real pirates vs. the temperature, the correlation runs the other way (temperature affects weather affects seafaring activities). I have no idea what causes the correlation between searches for free movies and searches about global warming; perhaps some third factor. To give another silly example, there is a lot more traffic on the roads during daylight than at night, but this isn't because cars are solar-powered!

The point is that experiments don't have this problem: you go out and manipulate the world to see what happens. Change the number of pirates and see if global temperatures change. Nobody has tried this (to my knowledge), but I'm willing to bet it won't work.

(Of course, there are natural experiments, which are a hybrid of observational studies and experiments: the experimenter doesn't manipulate the world herself but rather waits until somebody else, in the course of normal events, does it for her. Good examples are comparing different states as they adopt bicycle helmet laws at different times and comparing that against head injury stats in the various states. These are rarely as well-controlled as an actual experiment, but have the advantage of ecological validity.)

Reason #2: Life's too short

The second is that observational studies are limited by what actually happens in the world. You won't, from an observational study, find out what the effect of US politics is of every US senator taking up crack while every US representative takes up meth. (I hope not, anyway.)

That was an absurd example, but the problem is real. Language gives lots of great examples. Suppose you want to find out what sentences in any given language are grammatical and what sentences are not. You could do an observational study and see what sentences people say. Those are grammatical; sentences you haven't heard probably aren't.

The problem with this is that people are boring and repetitive. A small number of words (heck, a small number of sentence fragments) accounts for most of what people say and write. The vast majority of grammatical sentences will never appear in your observational sample no matter how long you wait, because there are actually an infinite number of grammatical English sentences. (In his impressive "Who's afraid of George Kingsley Zipf?", Charles Yang shows how a number of prominent language researchers went astray by paying too much attention to this kind of observational study.)

The basic feature of the problem is that for building theories -- explaining why things are the way they are -- very often what you care about are the border cases. Human behavior is largely repetitive, and the  border cases are quite rare. Experiments turn this around: by deliberately choosing the situations we put our participants in, we can focus on the informative test cases.

The experimental method: Here to stay

None of this should be taken as meaning that I don't think observational studies are useful. I conduct them myself. A prerequisite to asking the question Why are things the way they are is knowing, in fact, what way things are. There is also the question of ecological validity. When we conduct laboratory experiments, we construct artificial situations and then try to generalize the results to real life. It's good to know something about real life in order to inform those generalizations.

But just as I can't imagine observational studies disappearing, I can't imagine them replacing experimentation, either.