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help for collapse2}
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Augmented version of collapse to make dataset of means, medians, etc.

        collapse2 clist [weight] [if exp] [in range] [, by(varlist) cw fast]


    where clist is either

        [(stat)] varlist [ [(stat)] ... ]
        [(stat)] target_var=varname [target_var=varname ...] [ [(stat)] ...]

    or any combination of the varlist or target_var forms, and stat is one of

        first       first
        last        last
        firstnm     first non-missing
        lastnm      last non-missing

        sd          standard deviations
        sum         sums
        rawsum      sums ignoring optionally specified weight

        count       number of nonmissing observations
        max         maximums
        min         minimums

        median      medians
        p1          1st percentile
        p2          2nd percentile
        ...         3rd -- 49th percentiles
        p50         50th percentile (same as median)
        ...         51st -- 97th percentiles
        p98         98th percentile
        p99         99th percentile
        iqr         interquartile range

    If stat is not specified, mean is assumed.

    aweights, fweights, pweights, and iweights are allowed; see help weights
    and see note concerning weights below.  Statistic sd (standard deviation)
    is not allowed with pweights.

    varlist and varname in clist may contain time-series operators; see help 
    varlist.


Description

    collapse2 is an extension of Stata's built-in collapse command, which
    converts the data in memory into a dataset of means, sums, medians, etc.
    It differs only in offering four additional "aggregation" operators:
    first, last, firstnm, and lastnm, described above.  These are useful, for
    example, if you want to create a cross-section data set from annual data,
    with initial population as a variable.  You must first tsset your data to
    use these additional operators.

    For most details on the command, see help collapse.

Examples

    . collapse2 (first) startpop=pop (last) endpop=pop, by(country)

    . collapse2 (firstnm) startknownpop=pop (lastnm) endknownpop=pop,
        by(country)

    * Extract first available observation for pop and year it is for.
    . gen popdatastart = year if pop < .
    . collapse2 (firstnm) popdatastart startpop=pop, by(country)

Author

    David Roodman
    Center for Global Development
    Washington, DC
    droodman@cgdev.org

Also see

    Manual:  [R] collapse

    Online:  help for collapse, contract, egen, statsby, summarize, tabdisp, 
             table