Reshape Grouped Data

Description

This function reshapes a data frame between ‘wide’ format (with repeated measurements in separate columns of the same row) and ‘long’ format (with the repeated measurements in separate rows).

Usage

reshape(data, varying = NULL, v.names = NULL, timevar = "time",
        idvar = "id", ids = 1:NROW(data),
        times = seq_along(varying[[1]]),
        drop = NULL, direction, new.row.names = NULL,
        sep = ".",
        split = if (sep == "") {
            list(regexp = "[A-Za-z][0-9]", include = TRUE)
        } else {
            list(regexp = sep, include = FALSE, fixed = TRUE)}
        )

### Typical usage for converting from long to wide format:

# reshape(data, direction = "wide",
#         idvar = "___", timevar = "___", # mandatory
#         v.names = c(___),    # time-varying variables
#         varying = list(___)) # auto-generated if missing

### Typical usage for converting from wide to long format:

### If names of wide-format variables are in a 'nice' format

# reshape(data, direction = "long",
#         varying = c(___), # vector 
#         sep)              # to help guess 'v.names' and 'times'

### To specify long-format variable names explicitly

# reshape(data, direction = "long",
#         varying = ___,  # list / matrix / vector (use with care)
#         v.names = ___,  # vector of variable names in long format
#         timevar, times, # name / values of constructed time variable
#         idvar, ids)     # name / values of constructed id variable

Arguments

Details

Although reshape() can be used in a variety of contexts, the motivating application is data from longitudinal studies, and the arguments of this function are named and described in those terms. A longitudinal study is characterized by repeated measurements of the same variable(s), e.g., height and weight, on each unit being studied (e.g., individual persons) at different time points (which are assumed to be the same for all units). These variables are called time-varying variables. The study may include other variables that are measured only once for each unit and do not vary with time (e.g., gender and race); these are called time-constant variables.

A ‘wide’ format representation of a longitudinal dataset will have one record (row) for each unit, typically with some time-constant variables that occupy single columns, and some time-varying variables that occupy multiple columns (one column for each time point). A ‘long’ format representation of the same dataset will have multiple records (rows) for each individual, with the time-constant variables being constant across these records and the time-varying variables varying across the records. The ‘long’ format dataset will have two additional variables: a ‘time’ variable identifying which time point each record comes from, and an ‘id’ variable showing which records refer to the same unit.

The type of conversion (long to wide or wide to long) is determined by the direction argument, which is mandatory unless the data argument is the result of a previous call to reshape. In that case, the operation can be reversed simply using reshape(data) (the other arguments are stored as attributes on the data frame).

Conversion from long to wide format with direction = "wide" is the simpler operation, and is mainly useful in the context of multivariate analysis where data is often expected as a wide-format matrix. In this case, the time variable timevar and id variable idvar must be specified. All other variables are assumed to be time-varying, unless the time-varying variables are explicitly specified via the v.names argument. A warning is issued if time-constant variables are not actually constant.

Each time-varying variable is expanded into multiple variables in the wide format. The names of these expanded variables are generated automatically, unless they are specified as the varying argument in the form of a list (or matrix) with one component (or row) for each time-varying variable. If varying is a vector of names, it is implicitly converted into a matrix, with one row for each time-varying variable. Use this option with care if there are multiple time-varying variables, as the ordering (by column, the default in the matrix constructor) may be unintuitive, whereas the explicit list or matrix form is unambiguous.

Conversion from wide to long with direction = "long" is the more common operation as most (univariate) statistical modeling functions expect data in the long format. In the simpler case where there is only one time-varying variable, the corresponding columns in the wide format input can be specified as the varying argument, which can be either a vector of column names or the corresponding column indices. The name of the corresponding variable in the long format output combining these columns can be optionally specified as the v.names argument, and the name of the time variables as the timevar argument. The values to use as the time values corresponding to the different columns in the wide format can be specified as the times argument. If v.names is unspecified, the function will attempt to guess v.names and times from varying (an explicitly specified times argument is unused in that case). The default expects variable names like x.1, x.2, where sep = "." specifies to split at the dot and drop it from the name. To have alphabetic followed by numeric times use sep = "".

Multiple time-varying variables can be specified in two ways, either with varying as an atomic vector as above, or as a list (or a matrix). The first form is useful (and mandatory) if the automatic variable name splitting as described above is used; this requires the names of all time-varying variables to be suitably formatted in the same manner, and v.names to be unspecified. If varying is a list (with one component for each time-varying variable) or a matrix (one row for each time-varying variable), variable name splitting is not attempted, and v.names and times will generally need to be specified, although they will default to, respectively, the first variable name in each set, and sequential times.

Also, guessing is not attempted if v.names is given explicitly, even if varying is an atomic vector. In that case, the number of time-varying variables is taken to be the length of v.names, and varying is implicitly converted into a matrix, with one row for each time-varying variable. As in the case of long to wide conversion, the matrix is filled up by column, so careful attention needs to be paid to the order of variable names (or indices) in varying, which is taken to be like x.1, y.1, x.2, y.2 (i.e., variables corresponding to the same time point need to be grouped together).

The split argument should not usually be necessary. The split$regexp component is passed to either strsplit or regexpr, where the latter is used if split$include is TRUE, in which case the splitting occurs after the first character of the matched string. In the strsplit case, the separator is not included in the result, and it is possible to specify fixed-string matching using split$fixed.

Value

The reshaped data frame with added attributes to simplify reshaping back to the original form.

See Also

stack, aperm; relist for reshaping the result of unlist. xtabs and as.data.frame.table for creating contingency tables and converting them back to data frames.

Examples

summary(Indometh) # data in long format

## long to wide (direction = "wide") requires idvar and timevar at a minimum
reshape(Indometh, direction = "wide", idvar = "Subject", timevar = "time")

## can also explicitly specify name of combined variable
wide <- reshape(Indometh, direction = "wide", idvar = "Subject",
                timevar = "time", v.names = "conc", sep= "_")
wide

## reverse transformation
reshape(wide, direction = "long")
reshape(wide, idvar = "Subject", varying = list(2:12),
        v.names = "conc", direction = "long")

## times need not be numeric
df <- data.frame(id = rep(1:4, rep(2,4)),
                 visit = I(rep(c("Before","After"), 4)),
                 x = rnorm(4), y = runif(4))
df
reshape(df, timevar = "visit", idvar = "id", direction = "wide")
## warns that y is really varying
reshape(df, timevar = "visit", idvar = "id", direction = "wide", v.names = "x")


##  unbalanced 'long' data leads to NA fill in 'wide' form
df2 <- df[1:7, ]
df2
reshape(df2, timevar = "visit", idvar = "id", direction = "wide")

## Alternative regular expressions for guessing names
df3 <- data.frame(id = 1:4, age = c(40,50,60,50), dose1 = c(1,2,1,2),
                  dose2 = c(2,1,2,1), dose4 = c(3,3,3,3))
reshape(df3, direction = "long", varying = 3:5, sep = "")


## an example that isn't longitudinal data
state.x77 <- as.data.frame(state.x77)
long <- reshape(state.x77, idvar = "state", ids = row.names(state.x77),
                times = names(state.x77), timevar = "Characteristic",
                varying = list(names(state.x77)), direction = "long")

reshape(long, direction = "wide")

reshape(long, direction = "wide", new.row.names = unique(long$state))

## multiple id variables
df3 <- data.frame(school = rep(1:3, each = 4), class = rep(9:10, 6),
                  time = rep(c(1,1,2,2), 3), score = rnorm(12))
wide <- reshape(df3, idvar = c("school", "class"), direction = "wide")
wide
## transform back
reshape(wide)