Look at life satisfaction and success metrics, however you value them personally and from a source that you trust, conditioned on membership in the parties in y

Look at life satisfaction and success metrics, however you value them personally and from a source that you trust, conditioned on membership in the parties in your country. While half of everyone in a country must be unconditionally at or below average within the country that does not imply the same results hold conditional on party membership. I can't speak to the 'high cost' comment other than to suggest maybe look at medians in addition to modes.