{smcl}
{* *! version 0.0  2may2020}{...}
{viewerjumpto "Syntax" "manyweakiv##syntax"}{...}
{viewerjumpto "Description" "manyweakiv##description"}{...}
{viewerjumpto "Options" "manyweakiv##options"}{...}
{viewerjumpto "Examples" "manyweakiv##examples"}{...}
{viewerjumpto "Saved results" "manyweakiv##saved_results"}{...}
{viewerjumpto "Author" "manyweakiv##author"}{...}
{viewerjumpto "Acknowledgements" "manyweakiv##acknowledgements"}{...}
{title:Title}

{p2colset 5 19 21 2}{...}
{p2col :{hi:manyweakiv} {hline 2}}
implements the weak-identification robust jackknife AR test from Mikusheva and Sun (2022).  
The companying command {helpb manyweakivpretest} implements a new pre-test that is 
analogous to analogous to that of Stock and Yogo (2005) first stage F test,
 but robust to many instruments and heteroscedasticity.
{p_end}
{p2colreset}{...}
 
{marker syntax}{title:Syntax}

{p 8 15 2}
{cmd:manyweakivtest}
{y}
{cmd:(}{it:{help varlist:x}} {cmd:=}
        {it:{help varlist:instr}}{cmd:)} 
         [{it:{help varlist:covariates}}] {ifin}
{weight} {cmd:,} 
 [{it:options} {opt n:oconstant} ]
 
{p 8 15 2}
{cmd:manyweakivpretest}
{y}
{cmd:(}{it:{help varlist:x}} {cmd:=}
        {it:{help varlist:instr}}{cmd:)} 
         [{it:{help varlist:covariates}}] {ifin}
{weight} {cmd:,} 
 [{it:options} {opt n:oconstant} ]

{pstd}
where {it:x} is a scalar endogeneous variable.
{p_end} 
{synoptset 26 tabbed}{...}

{pstd}

{synopthdr :options}
{synoptline}
{syntab :Must specify}
{marker instr}{...}
{synopt :{opt instr}}specifies the list of (excluded) instruments.{p_end}

{syntab :Optional}
{synopt :{opt noconstant}}specifies whether an intercept is included (default includes an intercept).{p_end}
{synopt :{opt covariates}}specifies the list of controls, i.e., included instruments. {p_end}

{syntab :Saved Output}
{pstd}
{opt manyweakivtest} reports the jackknife AR confidence interval via analytical test inversion.  
{opt manyweakivpretest} reports the many-instruments F test.  
In addition, it stores the following in {cmd:e()}:

{synoptset 24 tabbed}{...}

{syntab:Scalar}
{synopt:{cmd:r(F)}}the many-instruments F statistic{p_end}

 
{marker description}{...}
{title:Description}

{pstd}
In empirical applications using instrumental variables, the current consensus 
practice is to report the first stage F statistic and as long as it is above 10, 
researchers are allowed to rely on standard t-statistics inferences. 
This practice has foundations in Stock and Yogo (2005) which showed that the 
concentration parameter fully characterizes the size distortion of the TSLS-Wald test, 
and empirically the concentration parameter can be judged based on the first stage F statistics. 
This result has been obtained under the assumptions of homoscedasticity and 
for a fixed number of instruments.

{pstd}
Mikusheva and Sun (2022) introduces a new F test that is valid under heteroscedasticity and many instruments.  Based on the result of this new F test (implemented in {opt manyweakivpretest}), applied researchers can switch between the 5% JIVE t-statistic or 5% jackknife AR test (implemented in {opt manyweakivtest}) with the caveats analogous to Stock and Yogo (2005):  Namely, the size of the two-step procedure are bounded within 15%.

{marker examples}{...}
{title:Examples}

{pstd}Simulate group instruments.{p_end}
{phang2}. {stata clear all}{p_end}
{phang2}. {stata set obs 100}{p_end}
{phang2}. {stata gen random  = uniform()}{p_end}
{phang2}. {stata gen group = 0}{p_end}
{phang2}. {stata local k = 10}{p_end}
	{cmd:forval j = 1/`k' {c -(}}
	{cmd:	replace group = `j' if random > (`j'-1)/`k' & random < (`j')/`k' }
	{cmd:{c )-}}
{phang2}. {stata tab group, gen(g_)}{p_end}
{phang2}. {stata gen v = rnormal()}{p_end}
{phang2}. {stata gen w = rnormal()}{p_end}
{phang2}. {stata gen x = 1 + w + v}{p_end}
{phang2}. {stata gen y = 1*x + w + 0.5*v}{p_end}	
	
{pstd} We use many-instrument F test to assess instruments' strength.  As expected we have weak instruments.{p_end}
{phang2}. {stata manyweakivpretest y (x = g_*) w}

{pstd} Simulate the outcome for illustrating the jackknife AR test, which as expected return unbounded confidence interval.{p_end}
{phang2}. {stata manyweakivtest y (x = g_*) w}{p_end}

{marker acknowledgements}{...}
{title:Acknowledgements}
  
{pstd}Thank you to the users of early versions of the program who devoted time to reporting
the bugs that they encountered.
 
{marker references}{...}
{title:References}
 

{marker MS2022}{...}
{phang}
Anna Mikusheva, Liyang Sun, Inference with Many Weak Instruments, The Review of Economic Studies, Volume 89, Issue 5, October 2022, Pages 2663–2686, Preprint.

{marker MS2023}{...}
{phang}
Mikusheva, A. and Sun, L. 2023. 
Weak Identification with Many Instruments. arXiv:2308.09535 [econ.EM]
{p_end}


{marker citation}{...}
{title:Citation and Installation of manyweakiv}

{pstd}{opt manyweakiv} is not an official Stata command. It is a free contribution
to the research community, like a paper. Please cite it as such: {p_end}

{phang}Sun, L., 2023.
manyweakiv: weak-instruments robust test for linear IV regressions with many instruments.
{browse "https://github.com/lsun20/manyweakiv":https://github.com/lsun20/manyweakiv}.

{pstd}{opt manyweakiv} can be installed easily via the {helpb github} package, which is available on 
{browse "https://github.com/haghish/github":https://github.com/haghish/github}. {p_end}

{phang2}. {stata github install lsun20/manyweakiv }{p_end}
{phang2}. {stata github update manyweakiv }{p_end}

{marker author}{...}
{title:Author}

{pstd}Liyang Sun{p_end}
{pstd}liyang.sun@ucl.ac.uk{p_end}