An oil refinery northeast of San Francisco conducted a series of 31 daily measurements of the carbon monoxide levels arising from one of their stacks between April 16 and May 16, 1993. The measurements were submitted as evidence for establishing a baseline to the Bay Area Air Quality Management District (BAAQMD). BAAQMD personnel had also made 9 independent measurements of the carbon monoxide from this same stack over the period from September 11, 1990 to March 30, 1993.
In this case, the refinery had an incentive to overestimate carbon monoxide emissions. The data show that both the average and median carbon monoxide measurements by the refinery were higher than the measurements by the BAAQMD. A t-test of the significance of this difference is not appropriate, however. A line plot of the carbon monoxide measurement over time shows a cyclical pattern, violating the assumption of independently distributed observations.