Hi,

I'm new to the forum, so I'm just learning the ropes around here. English is not my first language, so do overlook the grammatical mistakes.

I have two datasets that I want to merge, MaternityDB.dta and AllowancesDB.dta. The data sets look like this (only excerpts):
COUNTRY_ISO CODE YEAR ML_WKS
AU 1 1960 0
AU 1 1961 0
AT 2 1960 0
COUNTRY_ISO CODE YEAR TFR
AU 1 1960 3.45
AU 1 1961 3.55
AT 2 1960 2.69
I used this code:
use MaternityDB.dta, clear
merge m:1 COUNTRY_ISO CODE YEAR using AllowancesDB.dta

I think is a bit hard to understand the difference between one-to-one, one-to-many and many-to-one merges, so I tried all of them. However, I got the same red message from Stata: variables COUNTRY_ISO CODE YEAR do not uniquely identify observations in the using data/master data (depending which of the merge alternatives I tried).

I have tried to understand what the message means, but my searches haven't given results. Do any of you know what I do wrong? I would much appreciate the help, and I hope I have given the information needed.

Kind regards,
Frøydis