xtabs {stats} | R Documentation |
Create a contingency table (optionally a sparse matrix) from cross-classifying factors, usually contained in a data frame, using a formula interface.
xtabs(formula = ~., data = parent.frame(), subset, sparse = FALSE, na.action, addNA = FALSE, exclude = if(!addNA) c(NA, NaN), drop.unused.levels = FALSE) ## S3 method for class 'xtabs' print(x, na.print = "", ...)
formula |
a formula object with the cross-classifying variables
(separated by |
data |
an optional matrix or data frame (or similar: see
|
subset |
an optional vector specifying a subset of observations to be used. |
sparse |
logical specifying if the result should be a
sparse matrix, i.e., inheriting from
|
na.action |
a function which indicates what should happen when
the data contain |
addNA |
logical indicating if |
exclude |
a vector of values to be excluded when forming the set of levels of the classifying factors. |
drop.unused.levels |
a logical indicating whether to drop unused
levels in the classifying factors. If this is |
x |
an object of class |
na.print |
character string (or |
... |
further arguments passed to or from other methods. |
There is a summary
method for contingency table objects created
by table
or xtabs(*, sparse = FALSE)
, which gives basic
information and performs a chi-squared test for independence of
factors (note that the function chisq.test
currently
only handles 2-d tables).
If a left hand side is given in formula
, its entries are simply
summed over the cells corresponding to the right hand side; this also
works if the lhs does not give counts.
For variables in formula
which are factors, exclude
must be specified explicitly; the default exclusions will not be used.
In R versions before 3.4.0, e.g., when na.action = na.pass
,
sometimes zeroes (0
) were returned instead of NA
s.
By default, when sparse = FALSE
,
a contingency table in array representation of S3 class c("xtabs",
"table")
, with a "call"
attribute storing the matched call.
When sparse = TRUE
, a sparse numeric matrix, specifically an
object of S4 class
dgTMatrix
from package
Matrix.
table
for traditional cross-tabulation, and
as.data.frame.table
which is the inverse operation of
xtabs
(see the DF
example below).
sparseMatrix
on sparse
matrices in package Matrix.
## 'esoph' has the frequencies of cases and controls for all levels of ## the variables 'agegp', 'alcgp', and 'tobgp'. xtabs(cbind(ncases, ncontrols) ~ ., data = esoph) ## Output is not really helpful ... flat tables are better: ftable(xtabs(cbind(ncases, ncontrols) ~ ., data = esoph)) ## In particular if we have fewer factors ... ftable(xtabs(cbind(ncases, ncontrols) ~ agegp, data = esoph)) ## This is already a contingency table in array form. DF <- as.data.frame(UCBAdmissions) ## Now 'DF' is a data frame with a grid of the factors and the counts ## in variable 'Freq'. DF ## Nice for taking margins ... xtabs(Freq ~ Gender + Admit, DF) ## And for testing independence ... summary(xtabs(Freq ~ ., DF)) ## with NA's DN <- DF; DN[cbind(6:9, c(1:2,4,1))] <- NA; DN tools::assertError(# 'na.fail' should fail : xtabs(Freq ~ Gender + Admit, DN, na.action=na.fail)) xtabs(Freq ~ Gender + Admit, DN) xtabs(Freq ~ Gender + Admit, DN, na.action = na.pass) ## The Female:Rejected combination has NA 'Freq' (and NA prints 'invisibly' as "") xtabs(Freq ~ Gender + Admit, DN, addNA = TRUE) # ==> count NAs ## Create a nice display for the warp break data. warpbreaks$replicate <- rep_len(1:9, 54) ftable(xtabs(breaks ~ wool + tension + replicate, data = warpbreaks)) ### ---- Sparse Examples ---- if(require("Matrix")) withAutoprint({ ## similar to "nlme"s 'ergoStool' : d.ergo <- data.frame(Type = paste0("T", rep(1:4, 9*4)), Subj = gl(9, 4, 36*4)) xtabs(~ Type + Subj, data = d.ergo) # 4 replicates each set.seed(15) # a subset of cases: xtabs(~ Type + Subj, data = d.ergo[sample(36, 10), ], sparse = TRUE) ## Hypothetical two-level setup: inner <- factor(sample(letters[1:25], 100, replace = TRUE)) inout <- factor(sample(LETTERS[1:5], 25, replace = TRUE)) fr <- data.frame(inner = inner, outer = inout[as.integer(inner)]) xtabs(~ inner + outer, fr, sparse = TRUE) })