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Effective Standard Deviation of the Errors
 Effective Systematic Error
 Effective Random Error
2 2 1 σ
N effective +Σ= Σ
Error in estimating the average
2 1 1σ σ 




 − =
N effective
de Boer H C and Heijmen B J 2001 A protocol for the reduction of
systematic patient setup errors with minimal portal
imaging workload Int. J. Radiat. Oncol. Biol. Phys. 50 1350–65