- I made a pooled dataset containing cross-sectional data from different years: 2007, 2011, and 2017. And with the pooled dataset I want to evaluate a policy impact.
- The policy change happened in 2011.
- Individuals born before 2000 are the control group, and those born in 2000 or later are the treatment group.
To evaluate the policy I wanted to do a Difference in Difference analysis, So, I run this code on STATA, to see the impact of a specific policy change.....
didregress (edu_attainment highest_edu_level max_year_of_edu) (interaction), group(treatment) time(post_policy)
Where,
edu_attainment : Refers to the educational attainment of the individual
highest_edu_level : Refers to the highest educational level of the individual
max_year_of_edu : Refers to the maximum year of education of the individual
treatment : Refers to the treatment group Variable
post_policy : Refers to the Post Policy Variable
interaction : Refers to an interaction term between the treatment group (treatment) and the post-policy period (post_policy)
After running the code on STATA I got this result
note: interaction omitted because of collinearity.
model is not identified
The treatment variable interaction was omitted because of collinearity.
How do I solve this issue?
Please try
https://stats.stackexchange.com/questions/567966/difference-in-difference-regression-with-pooled-cross-sectional-data-fixed-eff
or Stata FAQs
https://www.stata.com/support/faqs/
All the best
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