# [R] automatic chi-square grouping in R

Roger.Lee@hannover-re.com Roger.Lee at hannover-re.com
Fri Oct 11 10:41:19 CEST 2002

```I'm doing some chi-square tests, and I recall some arbitrary rule that says each band must have at least 5 events in order for the test to be meaningful. Is there some way to do the banding automagically in R ? For instance, in the following survdiff, I'm trying to see if ADL affects survival. But when ADL=3,5 and 6, the number observed is too little. Anyway for me to tell R how to group them ? Like "R, combine ADL=5 and ADL=6, and redo the test" ?

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Call:
survdiff(formula = Surv(days, status) ~ adl, data = nu)

N Observed Expected (O-E)^2/E (O-E)^2/V
adl=0 92        6     8.74   0.86134   1.17556
adl=1 38        5     3.41   0.74346   0.83435
adl=2 60        9    10.56   0.22975   0.39159
adl=3 44        4     5.22   0.28487   0.33978
adl=4 27        6     2.32   5.83153   6.30818
adl=5 31        3     3.12   0.00456   0.00506
adl=6 16        2     1.63   0.08385   0.08835

Chisq= 8.2  on 6 degrees of freedom, p= 0.226

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On a related note, is it possible to tell R to group together values, for instance, if I have age in my data ranging from 30-60, is it possible to tell R to convert all ages 30-35 into 32.5, all age from 36-40 into 37.5 ... etc ? I mean I can always do this in Excel before I feed the data into R, but it seems R must be able to do something like this. I just don't know where to begin looking in the manual for something like this ...

Thanks so much guys,

Roger

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