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123 lines
5.4 KiB
Plaintext
123 lines
5.4 KiB
Plaintext
{smcl}
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{* 03Nov2005}{...}
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{hline}
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help for {hi:asprvalue}{right:03Nov2005}
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{hline}
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{title:Predicted probabilities for models with alternative-specific variables}
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{p 8 15 2}{cmd:asprvalue} [{cmd:,}
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{cmd:x(}{it:variables_and_values}{cmd:)}
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{cmdab:r:est(}{it:stat}{cmd:)}
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{cmdab:b:ase(}{it:refcatname}{cmd:)}
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{cmdab:c:at(}{it:catnames}{cmd:)}
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{cmdab:s:ave}
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{cmdab:d:iff}
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{cmdab:br:ief}
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{p 4 4 2}
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where {it:variables_and_values} is an alternating list of variables
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and numeric values
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{p 4 4 2}
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{it:stat} is either mean or asmean (alternative-specific means for alternative-specific variables)
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{p 4 4 2}
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{cmd:asprvalue} is intended to be used to compute predicted probabilities for logit or probit models
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that can combine case- and alternative-specific variables. For these models, predicted probabilities
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depend on the values of the independent variables, which may or may not vary over the alternatives for
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a particular case. {cmd:asprvalue} allows you to specify the values of the independent variables and
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presents predicted probabilities for the different alternatives. The command presently works after
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{helpb clogit}, {helpb rologit}, or {helpb asmprobit}.
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{p 4 4 2}
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{cmd:IMPORTANT:} For {helpb clogit} and {helpb rologit} models, case-specific variables are specified
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by a set of interactions with dummy variables for the alternatives. {cmd:asprvalue} can only be used
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if these interaction variables are named {it:alternative_name}X{it:case_specific_varname}. In other
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words, if the dummy variables for the alternatives are named "car" and "bus" and a case-specific
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variable is "male", the interactions must be named "carXmale" and "busXmale". These names for the
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interactions correspond with the names used if the data have been arranged for estimation using
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the command {cmd:case2choice}. A capital "X" cannot be used in the names of any of the other
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variables in the model.
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{title:Options}
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{p 4 8 2}
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{cmd:save} saves current values of indepenent variables and predictions
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for computing changes using the diff option.
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{p 4 8 2}
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{cmd:diff} computes difference between current predictions and those
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that were saved.
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{p 4 8 2}
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{cmd:x()} sets the values of independent variables for calculating
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predicted values. For case-specific variables, the list must alternate
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variable names and values. For alternative-specific variables, the list
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may either be followed by a single value to be assigned to all alternatives
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or J values if there are J alternatives. For {helpb clogit} or {helpb rologit}
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, when J values are specified, these
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are assigned to the alternatives in the order they have been specified by
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{cmd:cat()} or in the estimation command, with the value to be assigned to the
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reference category being last. For {helpb asmprobit}, the different alternatives are specified
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using a single variable rather than a series of dummy variables, and values for
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alternative-specific variables should be ordered to correspond with the ascending
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values of the variable.
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{p 4 8 2}
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{cmd:rest()} sets the values for variables unspecified in {cmd:x()}. The default
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is {it:mean}, which holds all unspecified variables to their case-specific means.
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One can also specific "asmean", which holds unspecified alternative-specific
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variables to their alternative-specific means. For example, if "time" was an
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alternative-specific variable, {it:mean} would assign all alternatives the
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mean of "time" over all individuals and alternatives, while {it:asmean} would assign
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each alternative the mean of "time" for that alternative.
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{p 4 8 2}
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{cmd:base()} specifies the name of the base (reference) category. If this is not
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specified, "base" will be used to refer to this category in the output. This option
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should not be used after {helpb asmprobit}.
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{p 4 8 2}
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{cmd:cat()} specifies the names of the dummy variables in the model used to
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indicate different alternatives (the alternative-specific intercepts). {cmd:cat()} only
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needs to be specified if the model includes no case-specific variables, as otherwise
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this list is inferred from the names of the interaction terms for case-specific
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variables. The name of the reference category should not be included in {cmd:cat()}. This option
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should not be used after {helpb asmprobit}.
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{p 4 8 2}
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{cmd:brief} prints only limited output.
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{title:Examples}
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{p 4 4 2}
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{cmd:. use "http://www.stata-press.com/data/lfr/nomocc2.dta", clear}{break}
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{cmd:. gen busXhinc = bus*hinc}{break}
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{cmd:. gen trainXhinc = train*hinc}{break}
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{cmd:. gen busXpsize = bus*psize}{break}
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{cmd:. gen trainXpsize = train*psize}{break}
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{cmd:. clogit choice train* bus* time invc , group(id)}{break}
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{cmd:. asprvalue, x(time 600 invc 30 hinc 40 psize 0) base(car)}{break}
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{p 4 4 2}
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{cmd:. asprvalue, x(psize 0) base(car) save}{break}
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{cmd:. asprvalue, x(psize 1) base(car) dif}{break}
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{p 4 4 2}
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{cmd:. asprvalue, x(psize 0) base(car) rest(asmean) save}{break}
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{cmd:. asprvalue, x(psize 1) base(car) rest(asmean) dif}{break}
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{p 4 4 2}
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{cmd:. asprvalue, x(time 600 hinc 40 psize 1) base(car) save}{break}
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{cmd:. asprvalue, x(time 700 600 600 hinc 40 psize 1) base(car) dif}{break}
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{cmd:. asprvalue, x(time 600 700 600 hinc 40 psize 1) base(car) dif}{break}
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{cmd:. asprvalue, x(time 600 600 700 hinc 40 psize 1) base(car) dif}{break}
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{title:Authors}
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Jeremy Freese and J. Scott Long
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{browse www.indiana.edu/~jslsoc/spost.htm}
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spostsup@indiana.edu
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