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un. On the other hand, if test mining were to occur, it would not be a bad idea
to separate it into different areas.
What could be done, in the absence of multiple impacted sites, to
address the problem of impact assessment in a rigorous statistical way? In
1979, Roger Green 13 had suggested an approach to an easier and more
convincing test for an impact. The basic procedure was now called beforeafter-control-impact
(BACI) studies or analyses. To illustrate this approach,
Etter used diversity as the response variable, saying that it did not matter for
the sake of determining the sampling protocol whether this was a reasonable
choice or not. The populations being studied were varying in both space and
time but the investigators did not know this; they could just take samples at
some time before and some time after the impact. The data, which could be
plotted as dots on a graph, represented not a single sample but the mean of a
series of replicate samples in or around each location. Green had argued that
the sampling before could be compared to the sampling after for both the
control and impact sites to better identify whether there had been an impact.
This could be done, he had said, by looking for significant interaction between
the localities in time. There would have been no interaction if both locations
had responded in the same way to an external influence. On the other hand, a
significant interaction would have occurred if the two locations had responded
differently. It was expected that the two locations would differ spatially and
temporally, but Green’s suggestion was that the interaction -- the difference in
the change between before and after for the control relative to the impacted --
would indicate whether there had been an impact. The impacted site should
show a different kind of response to the anthropogenic influence.
This was one strategy, Etter continued, but it did not solve the
problems. It did not account for all of the spatial and temporal variability
among these locations. Changes might take place that had nothing to do with
the anthropogenic effect, but that fact would go unrecognised because there
was no other impacted site with which to compare. Again, one could err either
way, finding that there had been an impact when there had not been or that
there was no significant difference when in fact there had been one. The
conclusion depended on how the control populations and the potentially
impacted populations changed with respect to one another over time. There
still remained the statistical problem of deciding whether an impact had
occurred in a particular place.
In a couple of articles published in the mid 1980s, statisticians had
gone a step further in terms of these BACI studies, suggesting that a series of
samples be taken before and afterwards. This would provide a better
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