Three reasons why reengineering legacy software is difficult

It is our belief that reverse engineering is the principal contributor to the difficulty of software reengineering. This difficulty is manifest as cost, measured in engineering resources. In this paper we look at three dimensions of reverse engineering based on three metrics believed to be classes of contributing factors and based on previous research. There are various types of reengineering that may be differentiated by their principal product and ordered according to the amount of reverse engineering required. The three "reasons" in the title refer to metrics: we will examine reengineering in each of these types in terms of understandability, effort, and coupling, three metrics that have been shown both theoretically and empirically to correlate strongly with reengineerability.