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help for center
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Center (or standardize) variables

        center varlist [weight] [if exp] [in range] [, casewise
               { prefix(prefix) | generate(newvar) } theta(#|varname)
               standardize replace double nolabel addtolabel(string)
               meansave[(prefix|newvar)] sdsave[(prefix|newvar)] ]

    by ... : may be used with center; see help by.

    aweights and fweights are allowed; see help weights.


Description

    center centers (or standardizes) the variables in varlist. For each
    varname in varlist, a new variable c_varname is created containing the
    centered (or standardized) values of that variable.

    Use the by prefix to center/standardize groupwise.


Options

    addtolabel(string) may be used to specify text which is to be added to
        the end of the labels of the new variables. Defaults are
        " (centered)" and " (standardized)", respectively.

    casewise specifies that cases with missing values be excluded listwise,
        i.e., that the centering/standardization be based on the sample that
        is not missing for any of the variables in varlist. The default is to
        use all the nonmissing values for each variable.

    double enforces storage type double.

    generate(newvar) specifies the name of the new variable to be created.
        Note that center may only be applied to one variable at a time if
        generate() is specified.

    meansave[(prefix)] saves variables containing the means. The new
        variables will be named prefixvarname. The default prefix is m_. Note
        that the saved means will not be affected by theta(). If generate()
        is specified, meansave(newvar) saves the mean(s) in newvar.

    nolabel suppresses the assignment of labels to the new variables. The
        default is to use the label (or name) of the original variable with
        some text added to the end of it (see addtolabel).

    prefix(prefix) allows the user to supply a prefix for the
        centered/standardized variables. The default prefix is c_, thus the
        new variables will be named c_varname. prefix() is not allowed if
        generate() is specified.

    replace permits center to overwrite existing variables.

    sdsave[(prefix)] saves variables containing the standard deviations
        (allowed only if the standardize option is specified). The new
        variables will be named prefixvarname. The default prefix is sd_. If
        generate() is specified, sdsave(newvar) saves the standard
        deviation(s) in newvar.

    standardize creates a variable containing the standardized values (zero
        sample mean and unit sample variance). Default is to create a
        variable containing the centered values (zero sample mean).

    theta(#|varname) may be used for quasi-demeaning. Before subtraction, the
        means will be scaled by # or by the values of varname respectively.
        This is sometimes used in the context of panel data models (see
        Wooldridge, 2002:287, and the Methods and Formulas Section in [XT]
        xtreg).


Examples

        . sysuse auto

        . center mpg price weight

        . center mpg price weight, prefix(z_) standardize

        . bysort rep78: center mpg price weight, replace

        . center mpg, generate(mpg0)
        

References

    Wooldridge, J.M. 2002. Econometric Analysis of Cross Section and Panel
        Data. Cambridge (Mass.): The MIT Press.


Author

    Ben Jann, ETH Zurich, jann@soz.gess.ethz.ch

    I am grateful to Mark Schaffer, who provided the motivation for the theta
    option.


Also see

    Manual:  [R] summarize

    On-line:  help for summarize