Secondary education in Northern Ireland is divided between grammar and non-grammar schools. Grammar schools use tests to select the most academically gifted students. If you fail the tests you can't go to a grammar school.
The political party Sinn Fein were opposed to selection and in 2011 they abolished the 11+ test. However a number of schools and thousands of parents opposed the move. As a result instead of using one state controlled test there are now several privately run tests used by different schools. Using a dataset from Open data NI I looked for any change in the number of children going to Grammar schools. If Sinn Fein's policy had been successful we could expect to see a drop in the number of children attending grammar schools.
However the above diagram shows a decline in the number of children attending non-grammar secondary schools while grammar school numbers have remained constant.
Based on this analysis my conclusion is that Sinn Fein's policy failed to reduce the grammar school share of the secondary school student numbers.