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A before-after control-impact design compares temporal change at an affected site with change at one or more control sites. In a simple two-period description, its treatment contrast isThis is a difference-in-differences comparison, or a site-type-by-period interaction term. For example, responses falling from $12$ to $8$ at the affected site and from $10$ to $9$ at the control site give $\Delta =-3$: the affected site fell three units more than the comparison site.

The design addresses both a pre-existing site difference and a common temporal change. Its causal interpretation still requires a defensible comparison for how the affected site would have changed without the disturbance. Another site-specific event coinciding with the intervention can confound the contrast. Multiple control sites and repeated sampling before and after the disturbance help characterize natural variation, while the analysis must account for dependence through time.^[ref]

Several observations within one affected site do not become several independent impact-site replicates. Distinguishing the experimental unit from its observational units prevents pseudoreplication; a large within-site sample does not repair missing independent site replication.