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The merge command combines data sets by combining observations that have the same value of an identifier variable or variables, so the result has all the variables from both files. If the using data set adds more variables to the master data set and observations represent the same things in both data sets, then this is a job for a one-to-one merge. The warning you got about labels already being defined tells you those variables have value labels defined in both files, and you should make sure that they agree about what the values mean. Of course that assumes they actually measure the same thing in the same way. Make sure variables have the same name in both files before appending them, or append will treat them as different variables. If a variable only exists in one of the two data sets, observations from the other data set will have missing values for that variable.

Use append to combine them into a single data set: Suppose that instead of a single file containing our 2000 ACS sample you were given 2000_acs_part_acs_part2, with each file containing about half of the observations. The append command simply adds the using data set to the end of the master data set. Using append makes sense if the master data set and the using data set contain the same kinds of things, but not the same things. If the using data set adds more observations to the master data set, then this is a job for append. Adding level one units to existing level two units.We'll discuss the most common scenarios, which are: How you combine the two data sets depends on what the using data set adds to the master data set. If the resulting data set won't have a consistent, logical structure you probably need to rethink what you're doing. When you're done, you'll have a single data set in memory again.Īlways think through what the resulting data set should look like before combining two data sets. (Stata 16 allows you to load multiple data sets as separate frames, but we won't explore this new feature.) So when we talk about combining data sets we mean taking a data set that's in memory, what Stata calls the master data set, and combining it with a data set on disk, what Stata calls the using data set for reasons that will become obvious when you see the command syntax. Also, combining data sets will often force you to deal with problems in the data set structure. However, if you've misunderstood the structure of the data sets you can end up with a data set that makes no sense at all. This is part seven of Data Wrangling in Stata.Ĭombining data sets is a very common task, and one that's easy to do if you understand the structure of the data sets you are trying to combine.