The core instrumental variable assumptions are:
- Instrument relevance: the genetic variant changes educational attainment.
- Instrumental-variable independence: it is independent of unmeasured common causes of education and myopia, after any justified ancestry adjustment.
- The exclusion restriction: it affects myopia only through education, with no horizontal pleiotropy.
For a local average effect one additionally uses instrumental-variable monotonicity. Because the valid instrument supplies variation in education independent of the confounders and has no direct route to the outcome, the instrument-outcome association can be attributed to the education changed by the instrument without measuring every confounder.
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