addinby is a "cleaner" alternative version of merge, designed to reduce the lines of code in Stata do-files. It adds variables and/or values to existing observations in the dataset currently in memory (the master dataset) from a Stata-format dataset stored in the file filename (the using dataset), using a foreign key of variables specified by the keyvarlist to identify observations in the using dataset. The using dataset must be sorted by the variables in the keyvarlist, and these variables must identify observations in the using dataset uniquely. Unlike -merge-, -addinby- always preserves the master dataset in its original sorting order, and does not add any merge-status variables or additional observations. However, addinby checks that the foreign key uniquely identifies observations in the using dataset, and may optionally check that there are no unmatched observations in the master dataset, and/or check that there are no missing values in the foreign key variables in the master dataset.