Chapter 5 described the logic behind the BACI approach and chapter 7 described the four basic BACI-type analytical models and the relative strengths of inference each provides (Table 7.1). We can make strong inferences (those with least uncertainty) about the effects of human impacts by examining differences between control and impact locations before and after the onset of human activity, most especially when we have replication of these design elements. However, what happens when one or more BACI elements are entirely missing or when we have no replication? Perhaps the most common problem facing environmental managers is where putative impacts have already occurred, tens or even hundreds of years before, and there is no scope for planning a Before period. There may be no control locations because all suitable locations have suffered the same human activity in question. The latter problem is particularly common when modern human activities are spread over large spatial scales because, as indicated earlier, it reduces the potential numbers of places we can search for controls. How should we proceed in these circumstances?

We must recognize first that the difficulties created here are ones of increased inferential uncertainty, not which analytical model to apply. When one or more of the four elements are missing, we lack the information that would otherwise allow us to distinguish, with some confidence, those changes caused by human impacts from those caused by alternative (natural) phenomena (Table 9.1; and see chapter 5).