The amount of lead in a certain type of soil, when released by a standard extraction method,
averages 86 parts per million (ppm). A new extraction method is tried on 40 specimens of the
soil, yielding a mean of 83 ppm lead and a standard deviation of 10 ppm.
(a) Is there significant evidence at the 5% level that the new method frees less lead from the
soil? What about the 1% level?
(b) A critic argues that because of variations in the soil, the effectiveness of the new method is
confounded with characteristics of the particular soil specimens used. Briefly describe a better
data production design that
avoids this criticism.