Statistical calculation (5)
(Other definitions for error that I've seen before include "Blunder, slip" , "Faulty judgement" , "Mistake or mishap" , "Way of operating" , "A mistake or inaccuracy" .)
'statistical calculation' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mode that I've seen before include "Fashion in dome" , "Custom" , "Manner, style" , "Fashion; manner" , "Fashion; way of performing" .)