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{* 6oct2005}{...}
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help for {hi:ivvif}}
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{title:Augmented version of {cmd:vif}/{cmd:estat ivvif} to report variance inflation factors.}

{p 4 4 2}
{cmd:ivvif} accepts no arguments. It can run after {cmd:regress}, {cmd:ivreg}, and {cmd:ivreg2}.

{title:Description}

{p 4 4 2}
{cmd:ivvif} extends Stata's official {cmd:vif}/{cmd:estat vif} command, which reports variance inflation factors.
It differs in two ways. As well as working after {cmd: regress}, it can run after instrumented regressions done with {cmd:ivreg} or {cmd:ivreg2}.
In this case, it projects regressors onto instruments before computing VIFs.
In other words, it reports the VIFs for the second stage of two-stage least squares.
The other difference is that it makes the results available in a return matrix, named "vif", along with {cmd: vif}'s return macros, which can ease working with the results in Stata code. {p_end}

{title:Examples}

{p 4 8 2}{cmd:. ivreg n (w = k)}{p_end}
{p 4 8 2}{cmd:. ivvif}{p_end}

{p 4 8 2}{cmd:. reg n w k}{p_end}
{p 4 8 2}{cmd:. ivvif}{p_end}
{p 4 8 2}{cmd:. mat list r(vif)}{p_end}

{title:Author}

{p 4}David Roodman{p_end}
{p 4}Center for Global Development {p_end}
{p 4}Washington, DC{p_end}
{p 4}droodman@cgdev.org{p_end}

{title:Also see}
{p 4 13 2}
Online:  help for {help vif}.