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.
fences(xx, units = "ppm", display = TRUE)
name of the variable to be processed.
the units of measurement, options are: “pct”, “ppm”, “ppb”, “ppt”. The default is “ppm”.
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 current device display = FALSE
as the displayed results will be saved to a text file for subsequent use/editing and reference.
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.
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.
# NOT RUN {
## Make test data available
data(kola.o)
attach(kola.o)
## Display the fences computed for Cu
fences(Cu)
## Detach test data
detach(kola.o)
# }
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