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POHow to do median splits within factor levels in R?
 

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  1. POHow to do median splits within factor levels in R?
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    <p>Here I make a new column to indicate whether myData is above or below its median</p> <pre><code>### MedianSplits based on Whole Data #create some test data myDataFrame=data.frame(myData=runif(15),myFactor=rep(c("A","B","C"),5)) #create column showing median split myBreaks= quantile(myDataFrame$myData,c(0,.5,1)) myDataFrame$MedianSplitWholeData = cut( myDataFrame$myData, breaks=myBreaks, include.lowest=TRUE, labels=c("Below","Above")) #Check if it's correct myDataFrame$AboveWholeMedian = myDataFrame$myData &gt; median(myDataFrame$myData) myDataFrame </code></pre> <p>Works fine. Now I want to do the same thing, but compute the median splits within each level of myFactor.</p> <p>I've come up with this:</p> <pre><code>#Median splits within factor levels byOutput=by(myDataFrame$myData,myDataFrame$myFactor, function (x) { myBreaks= quantile(x,c(0,.5,1)) MedianSplitByGroup=cut(x, breaks=myBreaks, include.lowest=TRUE, labels=c("Below","Above")) MedianSplitByGroup }) </code></pre> <p>byOutput contains what I want. It categorizes each element of factors A, B, and C correctly. However I'd like to create a new column, myDataFrame$FactorLevelMedianSplit, that shows the newly-computed median split.</p> <p>How do you convert the output of the "by" command into a useful data-frame column?</p> <p>I think perhaps the "by" command is not R-like way to do this ... </p> <p><strong>Update</strong>:</p> <p>With Thierry's example of how to use factor() cleverly, and upon discovering the "ave" function in Spector's book, I've found this solution, which requires no additional packages.</p> <pre><code>myDataFrame$MediansByFactor=ave( myDataFrame$myData, myDataFrame$myFactor, FUN=median) myDataFrame$FactorLevelMedianSplit = factor( myDataFrame$myData&gt;myDataFrame$MediansByFactor, levels = c(TRUE, FALSE), labels = c("Above", "Below")) </code></pre>
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    1. COThe packageless solution to this is beautiful -- thanks!
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