rgr (version 1.1.7)

fences: Generate and Display Fence Values

Description

Function to generate fence values to support the selection of the upper and lower bounds of background variability, i.e. threshold(s) or action levels, when an obvious graphical solution is not visually recognizable.

Usage

fences(xx, units = "ppm", display = TRUE)

Arguments

xx

name of the variable to be processed.

units

the units of measurement, options are: pct, ppm, ppb, ppt. The default is ppm.

display

the default is to display the tabular output on the current device, i.e. display = TRUE. However, when the function is used by fences.summary and in order to suppress output to the curr

Details

The fence values are computed by several procedures both with and without a logarithic data transformation and with a logit transformation, together with the 98th percentile of the data for display. Fences are computed following Tukey's boxplot procedure, as median +/- 2 * MAD (Median Absolute Deviation), and mean +/- 2 * SD (Standard Deviation), see Reimann et al. (2005). It is essential that these estimates be viewed in the context of the graphical distributional displays, e.g., shape and its graphical components, gx.hist, gx.ecdf, cnpplt and bxplot, and if spatial coordinates for the sample sites are available map.eda7, map.eda8 and caplot. The final selection of a range for background or the selection of a threshold level needs to take the statistical and spatial distributions of the data into account. It is also necessary to be aware that it might be appropriate to have more than one background range/threshold in a study or survey (Reimann and Garrett, 2005). The presence of relevant information in the data frame may permit the data to be subset on the basis of that information for display with the tbplots, bwplots and gx.cnpplts functions. If these indicate that the medians and middle 50%s of the data are visibly different, multiple background ranges may be advisable.

References

Filzmoser, P., Hron, K. and Reimann, C., 2009. Univariate statistical analysis of environmental (compositional) data: Problems and possibilities. Science of the Total Environment, 407(1/3):6100-6108. Reimann, C. and Garrett, R.G., 2005. Geochemical background - Concept and reality. Science of the Total Environment, 350(1-3):12-27. Reimann, C., Filzmoser, P. and Garrett, R.G., 2005. Background and threshold: critical comparison of methods of determination. Science of the Total Environment, 346(1-3):1-16. Reimann, C., Filzmoser, P., Garrett, R. and Dutter, R., 2008. Statistical Data Analysis Explained: Applied Environmental Statistics with R. John Wiley & Sons, Ltd., 362 p.

See Also

fences.summary, ltdl.fix.df, remove.na, logit, expit

Examples

## Make test data available
data(kola.o)
attach(kola.o)

## Display the fences computed for Cu
fences(Cu)

## Detach test data
detach(kola.o)