Since those just above the cutoff would likely be unobservedly
more motivated than those just the below the cutoff in the presence of manipulation, we expect that any GPA manipulation will bias our estimates towards finding that academic probation hurts student performance.
This method is preferred to a cross-sectional approach that compares communities with hospital closures to communities without hospital closures if counties experiencing a closure are unobservedly
different from those counties that did not experience a closure.
They have moved, suddenly and almost unobservedly
, quite across the sky (which before was clear) from west to east.