For my PhD, I am using PISA data from 2000, 2003 & 2018. My research focuses on SES achievement inequalities so I am therefore using the ESCS index. For 2000, the ESCS index is not included in the student file and has to be merged with the student file. However, I load both datasets in R, the ESCS file has more than double the observations. How do I merge these files?

First, I made a subset of the PISA 2000 student file and kept the variables that I need.
Then I tried to merge this file with the ESCS file.

subset <- pisa[, c(“STIDSTD”, “pv1math”, “comlrn”, “COUNTRY”, “ST03Q01”, “ST02Q01”,
“immigrant”, “SCHOOLID”)]

data <-merge(subset, SES, by=c(“STIDSTD”, “SCHOOLID”))