R: Plot Two Cumulative Distribution Functions Based on Censored...

cdfCompareCensored {EnvStats}

R Documentation

Plot Two Cumulative Distribution Functions Based on Censored Data

Description

For one sample, plots the empirical cumulative distribution function (ecdf) along with a theoretical cumulative distribution function (cdf). For two samples, plots the two ecdf's. These plots are used to graphically assess goodness of fit.

Usage

  cdfCompareCensored(x, censored, censoring.side = "left",
    y = NULL, y.censored = NULL, y.censoring.side = censoring.side,
    discrete = FALSE, prob.method = "michael-schucany",
    plot.pos.con = NULL, distribution = "norm", param.list = NULL,
    estimate.params = is.null(param.list), est.arg.list = NULL,
    x.col = "blue", y.or.fitted.col = "black", x.lwd = 3 * par("cex"),
    y.or.fitted.lwd = 3 * par("cex"), x.lty = 1, y.or.fitted.lty = 2,
    include.x.cen = FALSE, x.cen.pch = ifelse(censoring.side == "left", 6, 2),
    x.cen.cex = par("cex"), x.cen.col = "red",
    include.y.cen = FALSE, y.cen.pch = ifelse(y.censoring.side == "left", 6, 2),
    y.cen.cex = par("cex"), y.cen.col = "black", digits = .Options$digits, ...,
    type = ifelse(discrete, "s", "l"), main = NULL, xlab = NULL, ylab = NULL,
    xlim = NULL, ylim = NULL)

Arguments

`x`	numeric vector of observations. Missing (`NA`), undefined (`NaN`), and infinite (`Inf`, `-Inf`) values are allowed but will be removed.
`censored`	numeric or logical vector indicating which values of `x` are censored. This must be the same length as `x`. If the mode of `censored` is `"logical"`, `TRUE` values correspond to elements of `x` that are censored, and `FALSE` values correspond to elements of `x` that are not censored. If the mode of `censored` is `"numeric"`, it must contain only `1`'s and `0`'s; `1` corresponds to `TRUE` and `0` corresponds to `FALSE`. Missing (`NA`) values are allowed but will be removed.
`censoring.side`	character string indicating on which side the censoring occurs. The possible values are `"left"` (the default) and `"right"`.
`y`	a numeric vector (not necessarily of the same length as `x`). Missing (`NA`), undefined (`NaN`), and infinite (`Inf`, `-Inf`) values are allowed but will be removed. The default value is `y=NULL`, in which case the empirical cdf of `x` will be plotted along with the theoretical cdf specified by the argument `distribution`.
`y.censored`	numeric or logical vector indicating which values of `y` are censored. This must be the same length as `y`. If the mode of `censored` is `"logical"`, `TRUE` values correspond to elements of `y` that are censored, and `FALSE` values correspond to elements of `y` that are not censored. If the mode of `censored` is `"numeric"`, it must contain only `1`'s and `0`'s; `1` corresponds to `TRUE` and `0` corresponds to `FALSE`. Missing (`NA`) values are allowed but will be removed. This argument is ignored when `y` is not supplied. The default value is `y.censored=NULL` since the default value of `y` is `y=NULL`.
`y.censoring.side`	character string indicating on which side the censoring occurs for the values of `y`. The possible values are `"left"` (the default) and `"right"`. This argument is ignored when `y` is not supplied. The default value is `y.censoring.side=censoring.side`.
`discrete`	logical scalar indicating whether the assumed parent distribution of `x` is discrete (`discrete=TRUE`) or continuous (`discrete=FALSE`; the default).
`prob.method`	character string indicating what method to use to compute the plotting positions (empirical probabilities). Possible values are `"kaplan-meier"` (product-limit method of Kaplan and Meier (1958)), `"nelson"` (hazard plotting method of Nelson (1972)), `"michael-schucany"` (generalization of the product-limit method due to Michael and Schucany (1986)), and `"hirsch-stedinger"` (generalization of the product-limit method due to Hirsch and Stedinger (1987)). The default value is `prob.method="michael-schucany"`. The `"nelson"` method is only available for `censoring.side="right"`. See the help file for `ecdfPlotCensored` for more explanation.
`plot.pos.con`	numeric scalar between 0 and 1 containing the value of the plotting position constant. When `y` is supplied, the default value is `plot.pos.con=0.375`. When `y` is not supplied, for the normal, lognormal, three-parameter lognormal, zero-modified normal, and zero-modified lognormal distributions, the default value is `plot.pos.con=0.375`. For the Type I extreme value (Gumbel) distribution (`distribution="evd"`), the default value is `plot.pos.con=0.44`. For all other distributions, the default value is `plot.pos.con=0.4`. See the help files for `ecdfPlot` and `qqPlot` for more information. This argument is used only if `prob.method` is equal to `"michael-schucany"` or `"hirsch-stedinger"`.
`distribution`	when `y` is not supplied, a character string denoting the distribution abbreviation. The default value is `distribution="norm"`. See the help file for `Distribution.df` for a list of possible distribution abbreviations. This argument is ignored if `y` is supplied.
`param.list`	when `y` is not supplied, a list with values for the parameters of the distribution. The default value is `param.list=list(mean=0, sd=1)`. See the help file for `Distribution.df` for the names and possible values of the parameters associated with each distribution. This argument is ignored if `y` is supplied or `estimate.params=TRUE`.
`estimate.params`	when `y` is not supplied, a logical scalar indicating whether to compute the cdf for `x` based on estimating the distribution parameters (`estimate.params=TRUE`) or using the known distribution parameters specified in `param.list` (`estimate.params=FALSE`). The default value is `TRUE` unless the argument `param.list` is supplied. The argument `estimate.params` is ignored if `y` is supplied.
`est.arg.list`	when `y` is not supplied and `estimate.params=TRUE`, a list whose components are optional arguments associated with the function used to estimate the parameters of the assumed distribution (see the Section Estimating Distribution Parameters in the help file Censored Data). For example, all functions used to estimate distribution parameters have an optional argument called `method` that specifies the method to use to estimate the parameters. (See the help file for `Distribution.df` for a list of available estimation methods for each distribution.) To override the default estimation method, supply the argument `est.arg.list` with a component called `method`; for example `est.arg.list=list(method="mle")`. The default value is `est.arg.list=NULL` so that all default values for the estimating function are used. This argument is ignored if `estimate.params=FALSE` or `y` is supplied.
`x.col`	a numeric scalar or character string determining the color of the empirical cdf (based on `x`) line or points. The default value is `x.col="blue"`. See the entry for `col` in the help file for `par` for more information.
`y.or.fitted.col`	a numeric scalar or character string determining the color of the empirical cdf (based on `y`) or the theoretical cdf line or points. The default value is `y.or.fitted.col="black"`. See the entry for `col` in the help file for `par` for more information.
`x.lwd`	a numeric scalar determining the width of the empirical cdf (based on `x`) line. The default value is `x.lwd=3*par("cex")`. See the entry for `lwd` in the help file for `par` for more information.
`y.or.fitted.lwd`	a numeric scalar determining the width of the empirical cdf (based on `y`) or theoretical cdf line. The default value is `y.or.fitted.lwd=3*par("cex")`. See the entry for `lwd` in the help file for `par` for more information.
`x.lty`	a numeric scalar determining the line type of the empirical cdf (based on `x`) line. The default value is `x.lty=1`. See the entry for `lty` in the help file for `par` for more information.
`y.or.fitted.lty`	a numeric scalar determining the line type of the empirical cdf (based on `y`) or theoretical cdf line. The default value is `y.or.fitted.lty=2`. See the entry for `lty` in the help file for `par` for more information.
`include.x.cen`	logical scalar indicating whether to include censored values in `x` in the plot. The default value is `include.x.cen=FALSE`. If `include.x.cen=TRUE`, censored values in `x` are plotted using the plotting character indicated by the argument `x.cen.pch` (see below). This argument is ignored if there are no censored values in `x`.
`x.cen.pch`	numeric scalar or character string indicating the plotting character to use to plot censored values in `x`. The default value is `x.cen.pch=2` (hollow triangle pointing up) when `x.censoring.side="right"`, and `x.cen.pch=6` (hollow triangle pointing down) when `x.censoring.side="left"`. See the R help file for `points` for an explanation of how plotting symbols are specified. This argument is ignored if `include.x.cen=FALSE`.
`x.cen.cex`	numeric scalar that determines the size of the plotting character used to plot censored values in `x`. The default value is the current value of the `cex` graphics parameter. See the entry for `cex` in the R help file for `par` for more information. This argument is ignored if `include.x.cen=FALSE`.
`x.cen.col`	numeric scalar or character string that determines the color of the plotting character used to plot censored values in `x`. The default value is `x.cen.col="red"`. See the entry for `col` in the R help file for `par` for more information. This argument is ignored if `include.x.cen=FALSE`.
`include.y.cen`	logical scalar indicating whether to include censored values in `y` in the plot. The default value is `include.y.cen=FALSE`. If `include.y.cen=TRUE`, censored values in `y` are plotted using the plotting character indicated by the argument `y.cen.pch` (see below). This argument is ignored if `y` is not supplied and/or there are no censored values in `y`.
`y.cen.pch`	numeric scalar or character string indicating the plotting character to use to plot censored values in `y`. The default value is `y.cen.pch=2` (hollow triangle pointing up) when `y.censoring.side="right"`, and `y.cen.pch=6` (hollow triangle pointing down) when `y.censoring.side="left"`. See the R help file for `points` for an explanation of how plotting symbols are specified. This argument is ignored if `include.y.cen=FALSE`.
`y.cen.cex`	numeric scalar that determines the size of the plotting character used to plot censored values in `y`. The default value is the current value of the `cex` graphics parameter. See the entry for `cex` in the R help file for `par` for more information. This argument is ignored if `include.y.cen=FALSE`.
`y.cen.col`	numeric scalar or character string that determines the color of the plotting character used to plot censored values in `y`. The default value is `y.cen.col="black"`. See the entry for `col` in the R help file for `par` for more information. This argument is ignored if `include.y.cen=FALSE`.
`digits`	when `y` is not supplied, a scalar indicating how many significant digits to print for the distribution parameters. The default value is `digits=.Options$digits`.
`type`, `main`, `xlab`, `ylab`, `xlim`, `ylim`, `...`	additional graphical parameters (see `lines` and `par`). In particular, the argument `type` specifies the kind of line type. By default, the function `cdfCompareCensored` plots a step function (`type="s"`) when `discrete=TRUE`, and plots a straight line between points (`type="l"`) when `discrete=FALSE`. The user may override these defaults by supplying the graphics parameter `type` (`type="s"` for a step function, `type="l"` for linear interpolation, `type="p"` for points only, etc.).

Details

When both x and y are supplied, the function cdfCompareCensored creates the empirical cdf plot of x and y on the same plot by calling the function ecdfPlotCensored.

When y is not supplied, the function cdfCompareCensored creates the emprical cdf plot of x (by calling ecdfPlotCensored) and the theoretical cdf plot (by calling cdfPlot and using the argument distribution) on the same plot.

Value

When y is supplied, cdfCompareCensored invisibly returns a list with components:

`x.ecdf.list`	a list with components `Order.Statistics` and `Cumulative.Probabilities`, giving coordinates of the points that have been plotted for the `x` values.
`y.ecdf.list`	a list with components `Order.Statistics` and `Cumulative.Probabilities`, giving coordinates of the points that have been plotted for the `y` values.

When y is not supplied, cdfCompareCensored invisibly returns a list with components:

`x.ecdf.list`	a list with components `Order.Statistics` and `Cumulative.Probabilities`, giving coordinates of the points that have been plotted for the `x` values.
`fitted.cdf.list`	a list with components `Quantiles` and `Cumulative.Probabilities`, giving coordinates of the points that have been plotted for the fitted cdf.

Note

An empirical cumulative distribution function (ecdf) plot is a graphical tool that can be used in conjunction with other graphical tools such as histograms, strip charts, and boxplots to assess the characteristics of a set of data. It is easy to determine quartiles and the minimum and maximum values from such a plot. Also, ecdf plots allow you to assess local density: a higher density of observations occurs where the slope is steep.

Chambers et al. (1983, pp.11-16) plot the observed order statistics on the y-axis vs. the ecdf on the x-axis and call this a quantile plot.

Censored observations complicate the procedures used to graphically explore data. Techniques from survival analysis and life testing have been developed to generalize the procedures for constructing plotting positions, empirical cdf plots, and q-q plots to data sets with censored observations (see ppointsCensored).

Empirical cumulative distribution function (ecdf) plots are often plotted with theoretical cdf plots to graphically assess whether a sample of observations comes from a particular distribution. More often, however, quantile-quantile (Q-Q) plots are used instead of ecdf plots to graphically assess departures from an assumed distribution (see qqPlotCensored).

Author(s)

Steven P. Millard (EnvStats@ProbStatInfo.com)

References

Chambers, J.M., W.S. Cleveland, B. Kleiner, and P.A. Tukey. (1983). Graphical Methods for Data Analysis. Duxbury Press, Boston, MA, pp.11-16.

Cleveland, W.S. (1993). Visualizing Data. Hobart Press, Summit, New Jersey, 360pp.

D'Agostino, R.B. (1986a). Graphical Analysis. In: D'Agostino, R.B., and M.A. Stephens, eds. Goodness-of Fit Techniques. Marcel Dekker, New York, Chapter 2, pp.7-62.

Gillespie, B.W., Q. Chen, H. Reichert, A. Franzblau, E. Hedgeman, J. Lepkowski, P. Adriaens, A. Demond, W. Luksemburg, and D.H. Garabrant. (2010). Estimating Population Distributions When Some Data Are Below a Limit of Detection by Using a Reverse Kaplan-Meier Estimator. Epidemiology 21(4), S64–S70.

Helsel, D.R. (2012). Statistics for Censored Environmental Data Using Minitab and R, Second Edition. John Wiley & Sons, Hoboken, New Jersey.

Helsel, D.R., and T.A. Cohn. (1988). Estimation of Descriptive Statistics for Multiply Censored Water Quality Data. Water Resources Research 24(12), 1997-2004.

Hirsch, R.M., and J.R. Stedinger. (1987). Plotting Positions for Historical Floods and Their Precision. Water Resources Research 23(4), 715-727.

Kaplan, E.L., and P. Meier. (1958). Nonparametric Estimation From Incomplete Observations. Journal of the American Statistical Association 53, 457-481.

Lee, E.T., and J.W. Wang. (2003). Statistical Methods for Survival Data Analysis, Third Edition. John Wiley & Sons, Hoboken, New Jersey, 513pp.

Michael, J.R., and W.R. Schucany. (1986). Analysis of Data from Censored Samples. In D'Agostino, R.B., and M.A. Stephens, eds. Goodness-of Fit Techniques. Marcel Dekker, New York, 560pp, Chapter 11, 461-496.

Nelson, W. (1972). Theory and Applications of Hazard Plotting for Censored Failure Data. Technometrics 14, 945-966.

USEPA. (2009). Statistical Analysis of Groundwater Monitoring Data at RCRA Facilities, Unified Guidance. EPA 530/R-09-007, March 2009. Office of Resource Conservation and Recovery Program Implementation and Information Division. U.S. Environmental Protection Agency, Washington, D.C. Chapter 15.

USEPA. (2010). Errata Sheet - March 2009 Unified Guidance. EPA 530/R-09-007a, August 9, 2010. Office of Resource Conservation and Recovery, Program Information and Implementation Division. U.S. Environmental Protection Agency, Washington, D.C.

Examples

  # Generate 20 observations from a normal distribution with mean=20 and sd=5,
  # censor all observations less than 18, then compare the empirical cdf with a
  # theoretical normal cdf that is based on estimating the parameters.
  # (Note: the call to set.seed simply allows you to reproduce this example.)

  set.seed(333)
  x <- sort(rnorm(20, mean=20, sd=5))
  x
  # [1]  9.743551 12.370197 14.375499 15.628482 15.883507 17.080124
  # [7] 17.197588 18.097714 18.654182 19.585942 20.219308 20.268505
  #[13] 20.552964 21.388695 21.763587 21.823639 23.168039 26.165269
  #[19] 26.843362 29.673405

  censored <- x < 18
  x[censored] <- 18

  sum(censored)
  #[1] 7

  dev.new()
  cdfCompareCensored(x, censored)

  # Clean up
  #---------
  rm(x, censored)

  #==========

  # Example 15-1 of USEPA (2009, page 15-10) gives an example of
  # computing plotting positions based on censored manganese
  # concentrations (ppb) in groundwater collected at 5 monitoring
  # wells.  The data for this example are stored in
  # EPA.09.Ex.15.1.manganese.df.  Here we will compare the empirical
  # cdf based on Kaplan-Meier plotting positions or Michael-Schucany
  # plotting positions with various assumed distributions
  # (based on estimating the parameters of these distributions):
  # 1) normal distribution
  # 2) lognormal distribution
  # 3) gamma distribution

  # First look at the data:
  #------------------------

  EPA.09.Ex.15.1.manganese.df
  #   Sample   Well Manganese.Orig.ppb Manganese.ppb Censored
  #1       1 Well.1                 <5           5.0     TRUE
  #2       2 Well.1               12.1          12.1    FALSE
  #3       3 Well.1               16.9          16.9    FALSE
  #4       4 Well.1               21.6          21.6    FALSE
  #5       5 Well.1                 <2           2.0     TRUE
  #...
  #21      1 Well.5               17.9          17.9    FALSE
  #22      2 Well.5               22.7          22.7    FALSE
  #23      3 Well.5                3.3           3.3    FALSE
  #24      4 Well.5                8.4           8.4    FALSE
  #25      5 Well.5                 <2           2.0     TRUE

  longToWide(EPA.09.Ex.15.1.manganese.df,
    "Manganese.Orig.ppb", "Sample", "Well",
    paste.row.name = TRUE)

  #         Well.1 Well.2 Well.3 Well.4 Well.5
  #Sample.1     <5     <5     <5    6.3   17.9
  #Sample.2   12.1    7.7    5.3   11.9   22.7
  #Sample.3   16.9   53.6   12.6     10    3.3
  #Sample.4   21.6    9.5  106.3     <2    8.4
  #Sample.5     <2   45.9   34.5   77.2     <2


  # Assume a normal distribution
  #-----------------------------

  # Michael-Schucany plotting positions:
  dev.new()
  with(EPA.09.Ex.15.1.manganese.df,
    cdfCompareCensored(Manganese.ppb, Censored))

  # Kaplan-Meier plotting positions:
  dev.new()
  with(EPA.09.Ex.15.1.manganese.df,
    cdfCompareCensored(Manganese.ppb, Censored,
      prob.method = "kaplan-meier"))


  # Assume a lognormal distribution
  #--------------------------------

  # Michael-Schucany plotting positions:
  dev.new()
  with(EPA.09.Ex.15.1.manganese.df,
    cdfCompareCensored(Manganese.ppb, Censored, dist = "lnorm"))

  # Kaplan-Meier plotting positions:
  dev.new()
  with(EPA.09.Ex.15.1.manganese.df,
    cdfCompareCensored(Manganese.ppb, Censored, dist = "lnorm",
      prob.method = "kaplan-meier"))


  # Assume a gamma distribution
  #----------------------------

  # Michael-Schucany plotting positions:
  dev.new()
  with(EPA.09.Ex.15.1.manganese.df,
    cdfCompareCensored(Manganese.ppb, Censored, dist = "gamma"))

  # Kaplan-Meier plotting positions:
  dev.new()
  with(EPA.09.Ex.15.1.manganese.df,
    cdfCompareCensored(Manganese.ppb, Censored, dist = "gamma",
      prob.method = "kaplan-meier"))

  # Clean up
  #---------
  graphics.off()

  #==========

  # Compare the distributions of copper and zinc between the Alluvial Fan Zone
  # and the Basin-Trough Zone using the data of Millard and Deverel (1988).
  # The data are stored in Millard.Deverel.88.df.

  Millard.Deverel.88.df
  #    Cu.orig Cu Cu.censored Zn.orig  Zn Zn.censored         Zone Location
  #1       < 1  1        TRUE     <10  10        TRUE Alluvial.Fan        1
  #2       < 1  1        TRUE       9   9       FALSE Alluvial.Fan        2
  #3         3  3       FALSE      NA  NA       FALSE Alluvial.Fan        3
  #.
  #.
  #.
  #116       5  5       FALSE      50  50       FALSE Basin.Trough       48
  #117      14 14       FALSE      90  90       FALSE Basin.Trough       49
  #118       4  4       FALSE      20  20       FALSE Basin.Trough       50

  Cu.AF <- with(Millard.Deverel.88.df,
    Cu[Zone == "Alluvial.Fan"])

  Cu.AF.cen <- with(Millard.Deverel.88.df,
    Cu.censored[Zone == "Alluvial.Fan"])

  Cu.BT <- with(Millard.Deverel.88.df,
    Cu[Zone == "Basin.Trough"])

  Cu.BT.cen <- with(Millard.Deverel.88.df,
    Cu.censored[Zone == "Basin.Trough"])

  Zn.AF <- with(Millard.Deverel.88.df,
    Zn[Zone == "Alluvial.Fan"])

  Zn.AF.cen <- with(Millard.Deverel.88.df,
    Zn.censored[Zone == "Alluvial.Fan"])

  Zn.BT <- with(Millard.Deverel.88.df,
    Zn[Zone == "Basin.Trough"])

  Zn.BT.cen <- with(Millard.Deverel.88.df,
    Zn.censored[Zone == "Basin.Trough"])


  # First compare the copper concentrations
  #----------------------------------------
  dev.new()
  cdfCompareCensored(x = Cu.AF, censored = Cu.AF.cen,
    y = Cu.BT, y.censored = Cu.BT.cen)


  # Now compare the zinc concentrations
  #------------------------------------
  dev.new()
  cdfCompareCensored(x = Zn.AF, censored = Zn.AF.cen,
    y = Zn.BT, y.censored = Zn.BT.cen)


  # Compare the Zinc concentrations again, but delete
  # the one "outlier".
  #--------------------------------------------------

  summaryStats(Zn.AF)
  #       N    Mean      SD Median Min Max NA's N.Total
  #Zn.AF 67 23.5075 74.4192     10   3 620    1      68

  summaryStats(Zn.BT)
  #       N  Mean      SD Median Min Max
  #Zn.BT 50 21.94 18.7044   18.5   3  90

  which(Zn.AF == 620)
  #[1] 38

  summaryStats(Zn.AF[-38])
  #            N    Mean     SD Median Min Max NA's N.Total
  #Zn.AF[-38] 66 14.4697 8.1604     10   3  50    1      67


  dev.new()
  cdfCompareCensored(x = Zn.AF[-38], censored = Zn.AF.cen[-38],
    y = Zn.BT, y.censored = Zn.BT.cen)

  #----------

  # Clean up
  #---------

  rm(Cu.AF, Cu.AF.cen, Cu.BT, Cu.BT.cen,
     Zn.AF, Zn.AF.cen, Zn.BT, Zn.BT.cen)
  graphics.off()

[Package EnvStats version 2.8.1 Index]