# aggregate {stats}

### Description

Splits the data into subsets, computes summary statistics for each, and returns the result in a convenient form.

### Usage

aggregate(x, ...) ## S3 method for class 'default': aggregate((x, ...)) ## S3 method for class 'data.frame': aggregate((x, by, FUN, ..., simplify = TRUE)) ## S3 method for class 'formula': aggregate((formula, data, FUN, ..., subset, na.action = na.omit)) ## S3 method for class 'ts': aggregate((x, nfrequency = 1, FUN = sum, ndeltat = 1, ts.eps = getOption("ts.eps"), ...))

### Arguments

- x
- an R object.
- by
- a list of grouping elements, each as long as the variables in the data frame
`x`

. The elements are coerced to factors before use. - FUN
- a function to compute the summary statistics which can be applied to all data subsets.
- simplify
- a logical indicating whether results should be simplified to a vector or matrix if possible.
- formula
- a formula, such as
`y ~ x`

or`cbind(y1, y2) ~ x1 + x2`

, where the`y`

variables are numeric data to be split into groups according to the grouping`x`

variables (usually factors). - data
- a data frame (or list) from which the variables in formula should be taken.
- subset
- an optional vector specifying a subset of observations to be used.
- na.action
- a function which indicates what should happen when the data contain
`NA`

values. The default is to ignore missing values in the given variables. - nfrequency
- new number of observations per unit of time; must be a divisor of the frequency of
`x`

. - ndeltat
- new fraction of the sampling period between successive observations; must be a divisor of the sampling interval of
`x`

. - ts.eps
- tolerance used to decide if
`nfrequency`

is a sub-multiple of the original frequency. - ...
- further arguments passed to or used by methods.

### Details

`aggregate`

is a generic function with methods for data frames and time series.

The default method, `aggregate.default`

, uses the time series method if `x`

is a time series, and otherwise coerces `x`

to a data frame and calls the data frame method.

`aggregate.data.frame`

is the data frame method. If `x`

is not a data frame, it is coerced to one, which must have a non-zero number of rows. Then, each of the variables (columns) in `x`

is split into subsets of cases (rows) of identical combinations of the components of `by`

, and `FUN`

is applied to each such subset with further arguments in `...`

passed to it. The result is reformatted into a data frame containing the variables in `by`

and `x`

. The ones arising from `by`

contain the unique combinations of grouping values used for determining the subsets, and the ones arising from `x`

the corresponding summaries for the subset of the respective variables in `x`

. If `simplify`

is true, summaries are simplified to vectors or matrices if they have a common length of one or greater than one, respectively; otherwise, lists of summary results according to subsets are obtained. Rows with missing values in any of the `by`

variables will be omitted from the result. (Note that versions of R prior to 2.11.0 required `FUN`

to be a scalar function.)

`aggregate.formula`

is a standard formula interface to `aggregate.data.frame`

.

`aggregate.ts`

is the time series method, and requires `FUN`

to be a scalar function. If `x`

is not a time series, it is coerced to one. Then, the variables in `x`

are split into appropriate blocks of length `frequency(x) / nfrequency`

, and `FUN`

is applied to each such block, with further (named) arguments in `...`

passed to it. The result returned is a time series with frequency `nfrequency`

holding the aggregated values. Note that this make most sense for a quarterly or yearly result when the original series covers a whole number of quarters or years: in particular aggregating a monthly series to quarters starting in February does not give a conventional quarterly series.

`FUN`

is passed to `match.fun`

, and hence it can be a function or a symbol or character string naming a function.

### Values

For the time series method, a time series of class `"ts"`

or class `c("mts", "ts")`

.

For the data frame method, a data frame with columns corresponding to the grouping variables in `by`

followed by aggregated columns from `x`

. If the `by`

has names, the non-empty times are used to label the columns in the results, with unnamed grouping variables being named `Group.<var>i</var>`

for `by[[<var>i</var>]]`

.

### References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) *The New S Language*. Wadsworth & Brooks/Cole.

### Examples

## Compute the averages for the variables in 'state.x77', grouped ## according to the region (Northeast, South, North Central, West) that ## each state belongs to. aggregate(state.x77, list(Region = state.region), mean) ## Compute the averages according to region and the occurrence of more ## than 130 days of frost. aggregate(state.x77, list(Region = state.region, Cold = state.x77[,"Frost"] > 130), mean) ## (Note that no state in 'South' is THAT cold.) ## example with character variables and NAs testDF <- data.frame(v1 = c(1,3,5,7,8,3,5,NA,4,5,7,9), v2 = c(11,33,55,77,88,33,55,NA,44,55,77,99) ) by1 <- c("red", "blue", 1, 2, NA, "big", 1, 2, "red", 1, NA, 12) by2 <- c("wet", "dry", 99, 95, NA, "damp", 95, 99, "red", 99, NA, NA) aggregate(x = testDF, by = list(by1, by2), FUN = "mean") # and if you want to treat NAs as a group fby1 <- factor(by1, exclude = "") fby2 <- factor(by2, exclude = "") aggregate(x = testDF, by = list(fby1, fby2), FUN = "mean") ## Formulas, one ~ one, one ~ many, many ~ one, and many ~ many: aggregate(weight ~ feed, data = chickwts, mean) aggregate(breaks ~ wool + tension, data = warpbreaks, mean) aggregate(cbind(Ozone, Temp) ~ Month, data = airquality, mean) aggregate(cbind(ncases, ncontrols) ~ alcgp + tobgp, data = esoph, sum) ## Dot notation: aggregate(. ~ Species, data = iris, mean) aggregate(len ~ ., data = ToothGrowth, mean) ## Often followed by xtabs(): ag <- aggregate(len ~ ., data = ToothGrowth, mean) xtabs(len ~ ., data = ag) ## Compute the average annual approval ratings for American presidents. aggregate(presidents, nfrequency = 1, FUN = mean) ## Give the summer less weight. aggregate(presidents, nfrequency = 1, FUN = weighted.mean, w = c(1, 1, 0.5, 1))

Documentation reproduced from R 3.0.2. License: GPL-2.