In: Statistics and Probability
Where does the square root of (1+1/n) factor come from in the prediction interval?
A tolerance interval includes two percentage values. What do the two percentage values represent?
Tolerance intervals are statistical ranges typically constructed
from on-site background data. Tolerance limits define the range of
data that fall within a specified percentage with a specified level
of confidence. The upper tolerance limit has been commonly used to
establish a background threshold value, however, prediction limits
are often favored for establishing a background threshold value in
groundwater because they account for repeated measures. An upper
tolerance limit (UTL) is designed to contain, but not exceed, a
large fraction (that is, 95%, 99%) of the possible background
concentrations, thus providing a reasonable upper limit on what is
likely to be observed in background. Similarly, the lower tolerance
limit (LTL) is designed to contain at most a certain percentage of
the possible background concentrations, thus providing a reasonable
lower limit on what is likely to be observed in background. The
fraction to be contained or ‘covered’ by the limit is the coverage
parameter, and must be specified along with a desired confidence
level. Tolerance limits explicitly account for the degree of
variation in the background population and the size of the sample
of measurements used to construct the limit.