Finally, regarding the studies which consider the effect of both levels of integration (internal and external) we have to mention: Vargas, Cardenas & Matarranz (2000), Stank, Keller & Daugherty (2001) and Gimenez & Ventura (2002). Stank, Keller & Daugherty (2001) and Gimenez & Ventura (2002) explored the contribution of both levels of integration simultaneously, while Vargas, Cardenas & Matarranz (2000) considered both levels of integration independently