One advantage of writing this blog for almost a decade is that there are a lots of topics I have already covered. However, software moving at the speed that it does, there are always updates.
So, today I’m going to recycle a couple of older posts that introduce you to propensity score matching. Then, tomorrow, I will show you how to get your propensity scores with just pointing and clicking with a FREE (as in free beer) version of SAS.
Before you even THINK about doing propensity score matching …
Propensity score matching has had a huge rise in popularity over the past few years. That isn’t a terrible thing, but in my not so humble opinion, many people are jumping on the bandwagon without thinking through if this is what they really need to do.
The idea is quite simple – you have two groups which are non-equivalent, say, people who attend a support group to quit being douchebags and people who don’t. At the end of the group term, you want to test for a decline in douchebaggery.
However, you believe that that people who don’t attend the groups are likely different from those who do in the first place, bigger douchebags, younger, and, it goes without saying, more likely to be male.
The very, very important key phrase in that sentence is YOU BELIEVE.
Before you ever do a propensity score matching program you should test that belief and see if your groups really ARE different. If not, you can stop right now. You’d think doing a few ANOVAs, t-tests or cross-tabs in advance would be common sense. Let me tell you something, common sense suffers from false advertising. It’s not common at all.
Even if there are differences between the groups, it may not matter unless it is related to your dependent variable, in this case, the Unreliable Measure of Douchebaggedness.
What type of Propensity Score Matching is for you? A statistics fable
Once upon a time there were statisticians who thought the answer to everything was to be as precise, correct and “bleeding edge” as possible. If their analyses were precise to 12 decimal places instead of 5, of course they were better because as everyone knows , 12 is more than 5 (and statisticians knew it better, being better at math than most people).
Occasionally, people came along who suggested that newer was not always better, that perhaps sentences with the word “bleeding” in them were not always reflective of best practices, as in,
“I stuck my hand in the piranha tank and now I am bleeding.”
Such people had their American Statistical Association membership cards torn up by a pack of wolves and were banished to the dungeon where they were forced to memorize regular expressions in Perl until their heads exploded. Either that, or they were eaten by piranhas.
Perhaps I am exaggerating a tad bit, but it is true that there has been an over-emphasis on whatever is the shiniest, new technique on the block. Before my time, factor analysis was the answer to everything. I remember when Structural Equation Modeling was the answer to everything (yes, I am old). After that, Item Response Theory (IRT) was the answer to everything. Multiple imputation and mixed models both had their brief flings at being the answer to everything. Now it is propensity scores.
A study by Sturmer et al. (2006) is just one example of a few recent analyses that have shown an almost logarithmic growth in the popularity of propensity score matching from a handful of studies to in the late nineties to everybody and their brother.
You can read the rest of the post about choosing a method of propensity score matching here. If your clicking finger is tired, the take away message is this — quintiles, which are much simpler, faster to compute and easier to explain, are generally just as effective as more complex methods.
Now that we are all excited about quintiles, the next couple of posts will show you how to compute those in a mostly pointy-clicky manner.