Hey Guys,

I am trying to fit a probit regression.
Where I use adtitional control variables such as bith year, squared birth year, country dummies and country-specific linear and quadratic trends in birth year.
Since I am new to Stata I'm havin troubles with squared birth year and country-specific linear and quadratic trends.
I don't know how to put it in my probit command, it always says omitted because of collinearity.

this is an example of my Data.
+-----------------------------------------+
mergeid yrbirth country
1. DE-234882-02 1954 Germany
2. FR-551652-01 1956 France
3. Bf-588731-01 1956 Belgium
4. Bf-946237-01 1956 Belgium
5. DE-280968-01 1956 Germany
6. IE-193642-01 1956 Ireland
7. PL-015077-01 1956 Poland
8. DE-934977-01 1956 Germany
9. CZ-825701-01 1956 Czech Republic
10. DK-805308-01 1954 Denmark
11. IE-900812-01 1956 Ireland
12. PL-399420-01 1956 Poland
13. ES-221311-01 1954 Spain
14. GR-047171-01 1956 Greece
15. ES-908555-01 1956 Spain

thank you in advance.