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Methods for assessing difference between groups in change when initial measurement is subject to intra‐individual variation
Author(s) -
Chambless Lloyd E.,
Roeback John R.
Publication year - 1993
Publication title -
statistics in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.996
H-Index - 183
eISSN - 1097-0258
pISSN - 0277-6715
DOI - 10.1002/sim.4780121304
Subject(s) - statistics , ordinary least squares , mathematics , estimator , variation (astronomy) , variance (accounting) , variable (mathematics) , observational study , econometrics , mathematical analysis , physics , accounting , astrophysics , business
Statistical analysis of difference between groups in change for some variable, adjusting for initial value, is complicated by the presence of intra‐individual variation in that variable. We estimate here the asymptotic bais that results from calculating the adjusted between‐group difference by ordinary least squares (OLS) from observed data. We also present explicit formulae that use the OLS estimates, the difference between treatment groups in mean initial values, and a measure of the intra‐individual variation to compute a corrected estimator and its variance. Alternatively, we can use OLS on transformed data to obtain unbiased estimates, in which we replace initial observed values by conditional Stein estimates of true values. We illustrate the results with data from an observational study and a clinical trial.