I came across an interesting SAS macro that was used for visualizing log odds relationships of data. This type of chart is helpful for visualizing the relationship between a binary dependent variable and a continuous independent variable. I don't use SAS on a daily basis as I prefer to use R. So I got to thinking that I could recreate this macro using only R. I thought this would be a good tutorial for R on developing functions, using different plot techniques, and overlapping chart types.

The following picture is the result of the logodds function in R. The chart is really close but not quite exact. For the histogram points I decided to use the default squares of the stripchart plot and used a grey color to make it look a little faded.

The following is the R script.

logoddsFnc <- function(data_ind, data_dep, ind_varname, min.count=1){

# Assumptions: x & y are numeric vectors of the same

# length, y is 0/1 varible. This returns a vector

# of breaks of the x variable where each bin has at

# least min.countnumber of y's

bin.by.other.count <- function(x, other, min.cnt=1) {

csum <- cumsum(tapply(other, x, sum))

breaks <- numeric(0)

i <- 1

breaks[i] <- as.numeric(names(csum)[1])

cursum <- csum[1]

for ( a in names(csum) ) {

if ( csum[a] - cursum >= min.cnt ) {

i <- i + 1

breaks[i] <- as.numeric(a)

cursum <- csum[a]

}

}

breaks

}

brks <- bin.by.other.count(data_ind, data_dep, min.cnt=min.count)

# Visualizing binary categorical data

var_cut <- cut(data_ind, breaks=brks, include.lowest=T)

var_mean <- tapply(data_dep, var_cut, mean)

var_median <- tapply(data_ind, var_cut, median)

mydf <- data.frame(ind=data_ind, dep=data_dep)

fit <- glm(dep ~ ind, data=mydf, family=binomial())

pred <- predict(fit, data.frame(ind=min(data_ind):max(data_ind)),

type="response", se.fit=T)

# Plot

plot(x=var_median, y=var_mean, ylim=c(0,1.15),

xlab=ind_varname, ylab="Exp Prob", pch=21, bg="black")

stripchart(data_ind[data_dep==0], method="stack",

at=0, add=T, col="grey")

stripchart(data_ind[data_dep==1], method="stack",

at=1, add=T, col="grey")

lines(x=min(data_ind):max(data_ind),

y=pred$fit, col="blue", lwd=2)

lines(lowess(x=var_median,

y=var_mean, f=.30), col="red")

lines(x=min(data_ind):max(data_ind),

y=pred$fit - 1.96*pred$se.fit, lty=2, col="blue")

lines(x=min(data_ind):max(data_ind),

y=pred$fit + 1.96*pred$se.fit, lty=2, col="blue")

}

logoddsFnc(icu$age, icu$died, "age", min.count=3)

The ICU data for this example can be found in the R package "vcdExtra". Special thanks to David of Univ. of Dallas for providing me with a way to develop breaks in the independent variable as seen by the bin.by.other.count function.

The author of the SAS macro is also the author of Visualizing Categorical Data by M. Friendly which is a great reference for analyzing and visualizing data in factored groups.

## 4 comments:

hi, thanks so much! It's interesting! One question: If I want to try it I need a package if want to obtain the data "e.g: data_ind). Isnt't it?

thanks in advance.

Martí Casals.

Yes you can get the data from the R package "vcdExtra". The data is named ICU. Also you can get the data from the Michael Friendly's VCD website but its not as clean and will require manipulation.

http://www.datavis.ca/sas/vcd/catdata/icu.sas

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