help dummies2
-------------------------------------------------------------------------------

Title

     todummies -- Create indicator variables from categorical variable or vice
versa


Syntax

     Create indicator variables from categorical variable

        todummies varname [levspec [levspec ...]] [if] [in] [ , to_options]


     where levspec is

        # ["varlabel"] | (numlist ["varlabel"])

     and parentheses and double quotes are required.


     Create categorical variable from indicator variables

        fromdummies varlist [if] [in] , generate(newvarname) [from_options]


Description

    todummies creates indicator variables (also called dummy variables) from
    one categorical variable.

    One indicator variable is created for each specified level of the
    categorical variable. Enclosing more than one level in parentheses,
    creates one dummy variable, indicating observations for which varname
    equals one of these values. Omitted levels will be coded as missing
    values in the created indicator variables. Omitting all levels, results
    in one indicator for each level of the original variable (see tabulate).

    min and max may be used as # and in numlist to refer to the minimum and
    maximum of varname. Consecutive # may be specified as a numlist of the
    form from/to (There is no space between from, / and to). There may not be
    more than 249 distinct values specified.

    Indicator variables will be labeled according to the value labels of the
    categorical variable, if no variable labels are specified. If varname has
    no value label attached, the dummies will not be labeled.


    fromdummies creates one categorical variable from binary indicator
    variables. It thus reverses

        tabulate varname ,generate(stubname)

    A value label is defined for newvarname, associating variable labels
    attached to variables in varlist with the corresponding values in
    newvarname.


Options

        +------------+
    ----+ to_options +-------------------------------------------------------

    generate(namelist|stub) specifies names for the indicator variables. If
        one stub is specified, dummies stub1, stub2, ... will be created. If
        generate is not specified, stub defaults to varname.

    reference(numlist) specifies values of varname to be used as the
        reference category. No additional indicator variable will be created.
        Instead, observations for which varname equals one of the values in
        numlist are coded 0 in all indicator variables. rest may be specified
        as reference, meaning all (nonmissing) values not specified in
        levspec. This option is ignored if no levels are specified.

    sic changes the default stub to varname as typed (possibly abbreviated),
        if generate is not specified. If only one indicator variable is
        created, sic suppreses numeric suffixes.

    novarlabel prevents labeling the indicator variables.


        +--------------+
    ----+ from_options +-----------------------------------------------------

    generate(newvarname) specifies the name for the categorical variable.
        This is a required option.

    reference creates the categorical variable, even if the dummies do not
        add up to 1. Observations for which all dummies are 0, are coded k +
        1, where k is the number of indicator variables.  The value label
        "reference" is added. fromdummies aborts with an error if the sum of
        the dummies is larger than 1.

    names in the created value label, associates variable names in varlist
        with corresponding values in newvarname.

    [no]vallabel[(lblname)] defines lblname as a value label for the
        categorical variable. Default lblname is newvarname. novallabel does
        not create a value label for the categorical variable.

    varlabel(name) specifies a variable label for the categorical variable.
        There is no default variable label.


Remarks 

    todummies vs. todummy

    todummies is useful to create indicator variables from one categorical
    variable, if some of the levels are to be collapsed.

    Say we have a variable, foo, with 5 levels: values 1 to 5. We want to
    create one binary variable, indicating level 1 and another indicator
    variable, representing levels 2 to 3. Level 4 is the reference, and level
    5 represents missing values.

    There are many ways to create our variables. Here is one:

        . recode foo (3 = 2)(5 = .), generate(bar)
        . tabulate bar ,generate(foobar)

    We can now use variables foobar1 and foobar2 in our analysis, omitting
    variable foobar3 as the reference.

    Here is how we use todummies to create the indicator variables

        . todummies foo 1 (2 3) ,reference(4) generate(foobar)

    Basically we collapsed the two lines of code above, into one line.  Note
    that foobar3 is not created in this example.

    We can also use todummy to create the indicator variables, coding

        . todummy foo if (foo != 5) ,values(1 \ 2 3 \ 4) stub(foobar)

    As with todummies, we have one line of code, but the need to specify an 
    if qualifier makes this approach less convenient.  The if qualifier makes
    sure that all observations with value 5 in variable foo are coded .
    (missing value) in the created indicator variables. Because specifying an
    if qualifier is easily forgotten, I recommend using todummies. Also, from
    a more technical point of view, todummy is very slow.


Examples

        . sysuse nlsw88
        . todummies race 2 (1 3) ,generate(black other)
        . fromdummies black other ,generate(race2)

        . todummies occ (1/2 "High occ.") 3/4 ,reference(9 10) sic
        . fromdummies occ? ,generate(occu) names reference


Acknowledgments

    dummies2 is inspired by dummies and dummieslab by Nick Cox and Philippe
    van Kerm.


Author

    Daniel Klein, University of Kassel, klein.daniel.81@gmail.com


Also see

    Online: tabulate, fvvarlist (Stata 11 an higher)

    if installed: dummies, dummieslab, todummy