1. SPSS Propensity Score Matching: An overview
AnnMaria De Mars
The Julia Group
Hi. Ph.d. Taught stats 27 years. Stat consultant 27 years . President. Pepperdine Prof. Blah blah blah

2. Let me tell you what we’re going to do …
Overview of PSM
Review logistic regression
PSM with quintiles
PSM with nearest neighbor
FUZZY match
I will probably go over everything twice, and yes, I know I am doing that. It’s like when you take statistics and read the textbook. It all seems to make sense until you try to do the problems. Or, you can do it in class when the professor is there and you go home and can’t because you forgot a step.

3. Why ?
Non-equivalent groups are being compared, e.g, hospitals with & without specialized trauma centers
Are differences due to patient characteristics or to hospital characteristics
Propensity score matching is used when non-equivalent groups are being compared. For example, hospitals with specialized trauma centers have higher mortality than other hospitals, however, they also see patients with more severe conditions.

4. What ?
Propensity score is the conditional probability of receiving a treatment given pre-treatment characteristics
The propensity score match can simultaneously control for several different variables on which treatment groups differ. So, given the condition that you are, say, a burn victim, under age 10 and female what is your probability of being seen in a trauma unit? What’s the probability of being seen in the trauma unit given the conditions that the patient is 16-25, male and has a gun shot wound

5. When?
Propensity scores are ideally used when there is substantial overlap in the groups. If those matched are the most extreme high scorers from one group and the most extreme low scorers from the second group then neither is representative and the results are suspect.
In this it is similar to Analysis of Covariance. You can’t take a class of gifted children, another class of children with mental retardation, covary for IQ and compare them. The differences are just too great.

6. How?
Step one:
A logistic regression is performed with group as the dependent variable. A propensity score is created for each subject giving their likelihood of being in group one or group two.

7. Step Two
There are two options for step two. Either subjects can be grouped by scores, usually into quintiles, or matched by propensity scores.

8. Step Three: Quintiles
Perform a regular regression analysis with group (quintiles) as a covariate?
-No, not the best way. People do it when they have a small sample and are worried about losing data. But it’s not advisable.

9. Step Three, Option 2: Match
Match subjects on propensity scores & perform a regular regression analysis on this subsample
1,000 subjects from rural hospitals matched with 1,000 subjects from urban hospitals who have the same propensity score

10. So simple, a child can do it?
Two out of the three methods could be done with no syntax at all, just pointing and clicking
While there are a number of reasons you WANT to do the syntax, including saving it as a record, because it is easier to make changes, I am not one of those people who sneers at pointing and clicking your way through an analysis. I think that is a perfectly fine way to get used to new procedures. However, if you do that, I strongly recommend you copy and paste your syntax

11. Propensity Scores, Quintiles
Step
Task
SPSS commands 1
Create propensity scores
Logistic Regression (dependent = CITY)
Analyze > Regression > Logistic 2
Create quintiles
Frequencies
Analyze > Descriptive >Frequencies
Transform > Recode into different variables 3
Match quintiles
Select If
Use Complex Samples 4
Conduct analysis
Logistic regression with OUTCOME
as dependent

12. Propensity Scores, Matching
Step
Task
SPSS commands 1
Create propensity scores
Logistic Regression (dependent = CITY)
Analyze > Regression > Logistic 2
Create Dataset1
Select and Save Dataset1 Records
Data > Select Cases >If
File > Save (select variables) 3
Create Dataset2

13. PSM Continued Step Task SPSS commands Data > Sort Cases4
Sort dataset1
Data > Sort Cases 5
Sort dataset2
Select and Save Dataset1 Records
Data > Select Cases >If
File > Save (select variables)
Merge files

14. And that … … is the general idea
There are many options, not nearly all of which we will cover in the next few days, but now you have the general idea

15. It would help if you already knew this
A little about regression Something about SPSS syntax But, if not, oh well .