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122 lines
5.5 KiB
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122 lines
5.5 KiB
Plaintext
{smcl}
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{* 05Dec2005jf}
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{hline}
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help for {hi:case2alt}{right:03Nov2005}
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{hline}
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{p 4 4 2}
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{title:Convert data from one observation per case to one observation per alternative per case}
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{p 4 12 2}{cmd:case2alt} {cmd:,}
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{{cmdab:choice(}{it:varname}{cmd:)} | {cmdab:rank(}{it:stubname}{cmd:)}}
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[{cmdab:a:lt(}{it:stubnames}{cmd:)}
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{cmdab:casev:ars(}{it:varlist}{cmd:)}
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{cmdab:case(}{it:varname}{cmd:)}
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{cmdab:g:enerate(}{it:newvar}{cmd:)}
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{cmdab:rep:lace}
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{cmdab:altnum(}{it:varname}{cmd:)}
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{cmdab:non:ames}]
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{title:Description}
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{p 4 4 2}
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{cmd:case2alt} is intended to be used to configure data for the estimation of estimation
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of regression models for alternative-specific data, such as {helpb clogit},
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{helpb rologit} or {helpb asmprobit}. {cmd:case2alt} presumes that you have data
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where each observation corresponds to an individual case and that you want to convert
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the data to the form in which each observation corresponds to an alternative for a specific case.
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{p 4 4 2}
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Imagine that you have data with an outcome that has four alternatives, with values 1, 2, 3 and 8.
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{cmd:case2alt} will reshape the data so that there are n*4 observations. If you specify an
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identifying variable with the {cmd:casevars()} option, this variable will continue to identify
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unique cases; otherwise, new variable _id will identify cases.
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{p 4 4 2}
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A new variable, called either _altnum or the name specified in {cmd:altnum()},
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will identify the alternatives within a case. Additionally, however, {cmd:case2alt}
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will create dummy variables y{it:value} that also identify alternatives.
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In our example the new dummy variables y1, y2, y3 and y8 will be created.
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Interactions will also be created with these dummies and any variables
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specified in {cmd:case()}. For the variable educ, {cmd:case2alt} will create
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variables y1_educ, y2_educ, y3_educ, and y8_educ.
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{p 4 4 2}
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If we have simple choice variable, then {cmd:case2alt} will create an outcome
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variable based on {cmd:y()} that contains 1 in the observation corresponding to
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the selected alternative and 0 for other alternatives. We can specify the name of
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this new outcome variable using the {cmd:gen()} option, or we can have it be the
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same as {opt y()} using the {cmd:replace} option, or (by default) we
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can have it be named choice.
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{p 4 4 2}
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After using {cmd:case2alt}, we would be able to estimate models like {helpb clogit} by typing, e.g.,:
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{p 4 4 2}
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{cmd:. clogit choice y1* y2* y3*, group(_id)}
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{p 4 4 2}
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Alternative-specific variables are specified using the {cmd:alt()} option.
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The contents of {cmd:alt()} should be {it:stubnames}, corresponding to a series
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of variables that contain the alternative-specific values.
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Specifying {cmd:alt(time)}, in our example, would imply that there are
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variables time1, time2, time3 and time8 in our case-level data.
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{p 4 4 2}
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Case-specific variables are specified using the {cmd:casevars()} option,
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where the contents should be a {varlist}. Neither the outcome nor id variable
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should be included in {cmd:casevars()}.
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{p 4 4 2}
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If we have ranked data, we can specify the ranked outcome with the
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{cmd:rank()} outcome. The content of rank should again be a stubname.
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Specifying {cmd:rank(}rank{cmd:)} in our example would assume there are
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variables rank1, rank2, rank3, rank8 that contain the relevant information
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on the ranks of each alternative.
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{title:Options}
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{p 4 8 2}{opth choice(varname)} or {opth rank(stubname)} is required. {varname} is the
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variable that indicates the value of the selected alternative.
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In the case of ranked outcome, {it:stubname} with {opt rank()} will contain
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the stub for the names of variables that contain information about ranks of alternatives.
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{p 4 8 2}{opth case(varname)} indicates the variable, either existing or to be
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created, that identifies individual cases.
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If {varname} is unspecified, a new variable named _id will be created.
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{p 4 8 2}{cmd:alt(}{it:stubnames}{cmd:)} contains the {it:stubnames} for
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alternative-specific variables. This requires that variables {it:stubname}# exist
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for each value of an alternative.
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{p 4 8 2}{opth casevars(varlist)} contains the names of the case-specific variables
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(not including the id or outcome variable).
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{p 4 8 2}{opth gen:erate(newvar)} and {opt replace} are used to name the variable that
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contain 1 for the selected alternative and 0 for non-selected alternatives.
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The variable will be named {newvar} if {newvar} is specified; the name of
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the variable specified in {cmd: y()} if {opt replace} is specified; and will be named {it:choice} otherwise.
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In the case of ranked data, the ranks will be contained in variable specified as the
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stub in {opt yrank()} and {opt gen:erate()} or {opt replace} will be ignored.
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{p 4 8 2}{opth altnum(varname)} indicates the name of the new variable used to indicate
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the alternatives. _altnum will be used if altnum() is not specified.
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{p 4 8 2}{opt non:ames} indicates that the case-specific interactions should be named
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y# instead of using the value labels of the outcome variable.
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{title:Example}
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{p 4 4 2}{cmd:. use "http://www.stata-press.com/data/lfr/nomocc2.dta", clear}{break}
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{cmd:. mlogit occ white ed exper}{break}
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{cmd:. case2alt, choice(ed) casevars(white ed exper) replace nonames}{break}
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{cmd:. clogit occ y1* y2* y3* y4*, group(_id)}{break}
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{p 4 4 2}{cmd:. case2alt, rank(rank92) casevars(hntest) alt(rank04) case(id)}
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{title:Authors}
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Jeremy Freese and J. Scott Long
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www.indiana.edu/~jslsoc/spost.htm
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spostsup@indiana.edu
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