Testing gravity with E_G: mapping theory onto observations

We present a complete derivation of the observationally motivated definition of the modified gravity statistic E_G. Using this expression, we investigate how variations to theory and survey parameters may introduce uncertainty in the general relativistic prediction of E_G. We forecast errors on E_G for measurements using two combinations of upcoming surveys, and find that theoretical uncertainties may dominate for a futuristic measurement. Finally, we compute predictions of E_G under modifications to general relativity in the quasistatic regime, and comment on the pros and cons of using E_G to test gravity with future surveys