# [R] re: pooling categories in a table

Michael Friendly friendly at yorku.ca
Tue Apr 9 15:34:27 CEST 2002

```Perhaps I should have given an example.

#  5 column matrix:  Day, Time, Station, State, Freq
tv.matrix<-read.table("C:/R/mosaics/tv.dat")

> tv.matrix[1:5,]
V1 V2 V3 V4 V5
1  1  1  1  1  6
2  2  1  1  1 18
3  3  1  1  1  6
4  4  1  1  1  2
5  5  1  1  1 11
>

tv <- array(tv.matrix[,5], dim=c(5,11,5,3))
dimnames(tv) <-
list(c("Monday","Tuesday","Wednesday","Thursday","Friday"),

c("8:00","8:15","8:30","8:45","9:00","9:15","9:30",
"9:45","10:00","10:15","10:30"),
c("A","C","N","F","Other"), c("O","S","P"))
names(dimnames(tv))<-c("Day", "Time", "Station", "State")

Say I want to collapse the 11 time categories to 3. The real problem
is when I have only the 4-way array (or an equivalent table).  But
here, I thought I could just collapse the 2nd column to 3 categories:

tv.matrix[,2] <- 1+ as.integer((tv.matrix[,2]-1) /4)
tv2 <- array(tv.matrix[,5],
dim=c(5,3,5,3))
dimnames(tv2) <-
list(c("Monday","Tuesday","Wednesday","Thursday","Friday"),
c("8:00-8:45","9:00-9:45","10:00-10:30"),
c("A","C","N","F","Other"), c("O","S","P"))
names(dimnames(tv2))<-c("Day", "Time", "Station", "State")

But something is wrong, because the margins are not the same:

> margin.table(tv,1)
Day
Monday   Tuesday Wednesday  Thursday    Friday
21271     20486     19304     19779     17275
> margin.table(tv2,1)
Day
Monday   Tuesday Wednesday  Thursday    Friday
996       912       962       898       771

What am I missing?

-Michael

--
Michael Friendly              friendly at yorku.ca
York University               http://www.math.yorku.ca/SCS/friendly.html
Psychology Department
4700 Keele Street             Tel:  (416) 736-5115 x66249
Toronto, Ontario, M3J 1P3     Fax:  (416) 736-5814
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```

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