dlimit: Impute Detection Limits

In USGS-R/smwrQW: Tools for censored data analysis

Impute Detection Limits

Description

Imputes values for missing detection limits for left-censored data.

Usage

dlimit(values, censor.codes, default = 1e-25)

Arguments

values	the numeric values.
censor.codes	logical or character, TRUE or "<" indicates left-censored at value and FALSE, " " or "" indicates a quantified value. Character codes "E" and "J" are censored at the current detection limit. Other characters are ignored and generate a warning.
default	the default detection limit value to assign to an uncensored value that is less than the currently imputed detection limit value.

Value

A vector of detection limits matched with each of the input values.

Warning

Tim Cohn, written communication, 5 Nov 2002 states that there are several problems with this approach:

1) for large data sets, it may be difficult for the user to group the observations into subsets as in the example above. If the observations are not grouped into subsets or the subsets do not have the censored observations as the first records, incorrect detection limits may be applied to the uncensored data.

2) for the case of multiple constituents, it may be impossible to place the censored observation within each subset at the top of the subset. Consider constituents Y and Z, each analyzed at Lab B, and each with one censored observation. The censored observations occur on different dates. In this case its not possible to have both observations as the first line in the subset from Lab B. (Note: the user could work around this problem by doing seperate runs for each constituent.)

3) When a subset of observations is completely uncensored, the detection limit from another subset will be applied. Consider the example above, but with the 10 observations from Lab B being completely uncensored. In this case, the detection limit from Lab A will be used for all 20 observations.

4) When all of the observations are uncensored (no '<' signs for a given constituent), there is no way for the user to specify the detection limit(s). In this case, a default value of 1.E-25 is used for all observations (consistent with ESTIMATOR).

Despite these problems, the approach is satisfactory for most applications: "the estimates are not very sensitive to the precise value of the censoring threshold for the above-threshold values."

Note

This function is based on the DLIMIT function in LOADEST (Runkel and others, 2004). The general assumption for imputing detection limits is that the data are sorted by time and the detection limits changes in a consistent pattern over time.

References

Runkel, R.L., Crawford, C.G., and Cohn, T.A., 2004, Load estimator (LOADEST) a FORTRAN program for estiamting constituent laods in streams and rivers: U.S. Gelogical Survey Techniques and Methods 4-A5, 69. Available online as https://pubs.er.usgs.gov/publication/tm4A5.

See Also

readNWQLdl

Examples

## The actual detection limits are 2,2,2,1,1,1.
dlimit(c(2,2,3,1,1,2), c(" ", "<", " ", " ", "<", " "))

USGS-R/smwrQW documentation built on Oct. 11, 2022, 6:13 a.m.