This is a classic matching problem, I believe. Based on your description, you
are looking for a 1-to-1 nearest-neighbor match based on the absolute
difference in dates, with the constraint that once a subject in File 2 is used,
all of their records are discarded.
Correct?
There are two classical approaches: greedy matching and global optimal
matching. While the greedy approach is simple to implement and often works, it
can sometimes get into trouble (i.e., run out of good matches). The optimal
approach would use the Hungarian algorithm (e.g., LSAP function in the clue
package) to find the combination of matches that results in the lowest total
date difference across the entire dataset.
Ravi
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