How to match with 3 different treatment groups?

[Ryan Lahooti]

Hello,
I am trying to match 3 groups (surg A, surg B, Surg C) by age and gender to if the surgery type has any effect on our variable of interest (ie weight loss). Is there a way to do this in stata? Thank you very much!

[Clyde Schechter]

Yes, it is possible. For a more helpful answer you need to:

1. Specify whether you want a 1:1:1 match or some other ratios.
2. Decide whether you want an exact match on age (usually results in large numbers of unmatched participants), or a "closest possible match," or a caliper match (and specify the width of the caliper).
3. Decide whether you want matching with or without replacement. Matching with replacement is statistically simplest, and also minimizes the number of unmatchable participants. However some people prefer matching without replacement on aesthetic grounds. (There are no statistical advantages to matching without replacement.
4. Show example data including participants from all three groups and including at least some matchable triplets. Be sure to use the -dataex- command to do that. If you are running version 18, 17, 16 or a fully updated version 15.1 or 14.2, -dataex- is already part of your official Stata installation. If not, run -ssc install dataex- to get it. Either way, run -help dataex- to read the simple instructions for using it. -dataex- will save you time; it is easier and quicker than typing out tables. It includes complete information about aspects of the data that are often critical to answering your question but cannot be seen from tabular displays or screenshots. It also makes it possible for those who want to help you to create a faithful representation of your example to try out their code, which in turn makes it more likely that their answer will actually work in your data.

[Ryan Lahooti]

Wow thank you!
1. Yes, 1,1,1
2. closest possible or caliper match +_3 years would be fine as well
3. Match with replacement if it is simplest and doesn't confer any disadvantage
4. I will work on this