The manufacturers’ published
accuracy standards for both the Multi-Pak and the ATX-620 quote accuracy as
percentage of measured CH4_LEL, rather than as an absolute
number. Thus, higher readings are expected to have higher variance. This was confirmed in the process of
computing the T statistics, when it was noticed that sample standard deviations
had a strong linear relationship to sample means. In such situations it is common practice to even out variance by
computing statistics on the logarithms of the original data. We followed this practice. Readings of zero percent of LEL were treated
as having a logarithm of –1. The sets
of increasing and decreasing sites computed by a T-test applied to the
logarithm were substantially the same as those computed by a T-test on the data
itself.