I want to use a risk score (RS) as an instrument for an exposure on a clinical outcome. However, I wont have access to data on the outcome for some time, and wish to examine whether this risk score would serve as a decent instrument. It is advised to report both the R-squared (as amount of variance explained by RS) and the F-statistic, but I'm having trouble understanding the following (basic) issues:
- Am I correct in assuming that R-squared is the squared semi-partial correlation, and is this simply the difference in R-squared with and without adding the instrument to the model in linear regression? (model would be: exposure ~ RS + age + sex)
- Running linear regression with the previously mentioned model, an F-statistic is produced. Is this the preferred F-estimate of instrument strength?