# qddplot: Quantile Difference Diagram In vudc: Visualization of Univariate Data for Comparison

## Description

Creates quantile difference diagrams. It takes two numerical datasets, sorts them, calculates differences on the same quantiles (i.e. lowest with the lowest, median with the median, 90% with 90% etc.) and displays these differences. It is appropriate for visual illustration of Wilcoxon rank test and F-test of equality of variances.

## Usage

 ```1 2 3 4 5``` ```qddplot(x, y, remove.ratio = 0.1, differences.range = NA, differences.rangemin = 10, differences.drawzero = TRUE, quantiles.drawhalf = TRUE, quantiles.showaxis = TRUE, line.lwd = 5, xlab = "Quantile", ylab = "Difference", main = "Quantile Differences", ...) ```

## Arguments

 `x, y` vectors of numeric values to be compared. It is not necessary to have the same length. `remove.ratio` a real number in range [0.0, 0.5). Indicates how much leading and tailing data should not be displayed in order to avoid a bias on the diagram caused by outliers. `differences.range` numeric, indicating the value range (i.e. the y axis) to be shown. The default value is NA, indicating the maximum absolute value is considered. This can be overridden with an explicit value. `differences.rangemin` numeric, indicating the minimum value of the value range. Used in combination with `differences.range=NA` (the default), else ignored. `differences.drawzero` logical, indicating if the y=0 helper line should be displayed. `quantiles.drawhalf` logical, indicating if the x=0.5 helper line should be displayed. `quantiles.showaxis` logical, indicating if the custom values on the x axis should be displayed. `line.lwd` width of the main line. `xlab` title for the x axis, see `plot`. `ylab` title for the y axis, see `plot`. `main` plot title, see `plot`. `...` standard graphic parameters. See `par` for details.

## Details

The quantile difference diagrams can be used to visualize the results of Wilcoxon rank tests (`wilcox.test`). Wilcoxon rank test basically checks if the values found in one dataset are different (more specifically: higher or lower) than those in the other. If the values in subset x are higher than values in subset y at all quantiles, then the result would look like a line completely on the same part of y coordinate (either completely on the positive or on the negative part).

In most of the cases the diagram resembles on a more or less straight line. The slope indicates the difference between variances (see also `var.test`). The position of the line indicates if the values in one dataset is higher than those in another; being close to 0 at 50%, if they are similar.

## Author(s)

Csaba Farago <farago@inf.u-szeged.hu>

## Examples

 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51``` ```# Using default settings with random data. qddplot(rnorm(100, 30, 50), rnorm(200, 10, 10)); # remove.ratio = 0.0 means the outliers are not removed. qddplot(rnorm(100, 30, 50), rnorm(200, 10, 10), remove.ratio=0.0); # remove.ratio = 0.25 means only the middle half is displayed, the upper and lower quantile are not. qddplot(rnorm(100, 30, 50), rnorm(200, 10, 10), remove.ratio=0.25); # Illustrating similar and different medians and variances on 4 quantile difference diagrams. # This is also an illustration for setting custom main title and subtitle. dataSetA <- seq(-20, 20) + rnorm(41); dataSetB <- seq(-15, 25) + rnorm(41); dataSetC <- seq(-40, 40) + rnorm(81); dataSetD <- seq(-20, 20) + rnorm(41); op <- par(mfrow=c(2,2)); qddplot( dataSetA, dataSetD, main = "Similar median, similar variance", sub = "-20...20 vs. -20...20"); qddplot( dataSetA, dataSetB, main = "Different median, similar variance", sub = "-20...20 vs. -15...25"); qddplot( dataSetA, dataSetC, main = "Similar median, different variance", sub = "-20...20 vs. -40...40"); qddplot( dataSetB, dataSetC, main = "Different median, different variance", sub = "-15...25 vs. -40...40"); par(op); # Change plot style: thicker line in red color. qddplot(rnorm(100, 30, 50), rnorm(200, 10, 10), line.lwd=10, col="red"); # Hide axes, helper lines and captions. qddplot(rnorm(100, 30, 50), rnorm(200, 10, 10), differences.drawzero=FALSE, quantiles.drawhalf=FALSE, quantiles.showaxis=FALSE, line.lwd=1, yaxt='n', main="", sub="", xlab="", ylab=""); # Do not consider minimal range. qddplot(rnorm(100, 1, 1), rnorm(100, 2, 1), differences.rangemin=NA); # Set an explicit range. qddplot(rnorm(100, 0, 200), rnorm(100, 0, 200), differences.range=40); ```

vudc documentation built on May 2, 2019, 9:19 a.m.