In this work, we present data envelopment analysis modeling approach for network data envelopment analysis where addition of efficiency is assumed for sub units or stages. The approach is applied under both constant return to scale and variable return to scale assumptions. We looked at the general multi stage processes classified as serial, parallel and non-immediate flow processes. The overall efficiency is expressed as a weighted sum of the efficiencies of the individual stages. We observe the more general problem of an open multistage process where some output from a given stage may leave the system while others become inputs to the next stage. We finally apply our approach to the numerical example.