Because of this lack of consistency, there is some confusion about which HR practices are the important ones in raising performance. There has also been debate about how practices should be combined (Delery, 1998). Some suggest that practices might be additive (combinations may produce additional and non-overlapping benefits), or substitutable (ie either on or off the job training will do and there may be no additional benefits in either)' synergistic ( more than the sum of the parts) or negatively synergistic (where certain combinations may be worse than no practice at all). It has been suggested that the positive results associated with much of the research on individual HR practices may be related to the underlying HR systems (Becker and Huselid, 1998)