{smcl}
{* 24 Dec 2014}{...}
{cmd:help hhi5}
{hline}

{title:Title}

{p2colset 5 14 22 2}{...}
{p2col :{hi:hhi5} {hline 2}}generates Herfindahl-Hirschman index (HHI) variables
{p_end}
{p2colreset}{...}


{title:Syntax}

{p 4 19 2}
{cmdab:hhi5} {varlist} {ifin}, {opth by:(varlist:groupvar)} 
[
{cmdab:pre:fix:}{cmd:(str)}
{cmdab:t:op:}{cmd:(#)}
{cmdab:mim:obs:}{cmd:(#)}
{opt per:centage}
{cmdab:out:file:}{cmd:(str)}
{opt replace}
{opt noe:xpand}
]


{title:Description}

{pstd}
hhi5 generates Herfindahl-Hirschman index (HHI) variables also commonly known as 
concentration index in economics and finance. {break}
See: {browse "http://baike.baidu.com/view/4362432.htm":http://baike.baidu.com/view/4362432.htm}.
The module supports multiple variables and group ids and 
allows user to export output in table format (csv comma delimited).


{title:Options}

{phang}
{opth by:(varlist:groupvar)} specifies the {it:groupvar} which defines the categories 
that {opt hhi5} will use to calculate the HHI index. For example,  
{opt by:(industry)}, {opt by:(industry year)}.

{phang}
{cmd:prefix(}{it:str}{cmd:)} prefixes new variable names with {it:str}. 
If {cmd:prefix(}{it:str}{cmd:)} is not specified, the defult prefix is "hhi_". 
Thus, {bf:hhi5 sale output, options} will generate 
two variables named as {it: hhi_sale} and {it: hhi_output}, respectively.

{phang}
{cmd:top(}{it:#}{cmd:)} use firms in each {it:groupvar} 
with the highest top # values of each variable in {varlist} to calculate the HHI index.

{phang}
{cmd:minobs(}{it:#}{cmd:)} specifies the minimum number of observations in each of the category defined by {it:groupvar}.

{phang}
{opt percentage} scale the market share of each firm in category defined by {it:groupvar} 
to be a percentage of the total, between 0 and 100. 
The defult is scaled to be a proportion of the total, between 0 and 1.
Specify this option cause the HHI index 10^4 times larger.

{phang}
{opt noexpand} restrict the HHI index values to sub-sample correponding {cmd:top(}{it:#}{cmd:)} or
{cmd:minobs(}{it:#}{cmd:)}. The defult is to expand the HHI index values to the full sample.

{phang}   
{cmd:outfile(}{it:filename}{cmd:)} export data in table format (.csv comma delimited), 
named as {it:filename}.csv.    


{title:Examples}

{result}{dlgtab:The nlsw88 data}{text}

{phang2}{inp:.} {stata "sysuse nlsw88,clear":sysuse nlsw88,clear}{p_end}
{phang2}{inp:.} {stata "gen sale = ln(wage)":gen sale = ln(wage)}{p_end}
{phang2}{inp:.} {stata "hhi5 sale, by(industry)":hhi5 sale, by(industry)}{p_end}
{phang2}{inp:.} {stata "hhi5 sale hour, by(industry race) prefix(hhi5) top(5) minobs(10)":hhi5 sale hour, by(industry race) prefix(hhi5) top(5) minobs(10) }{p_end}
{phang2}{inp:.} {stata "br industry race hhi*":br industry race hhi*}{p_end}


{title:Acknowledgements}

{pstd}   
Codes from {help hhi} by Muhammad Rashid Ansari 
({browse "mailto:rashid.ansari@insead.edu":rashid.ansari@insead.edu}) 
have been incorporated. 
{p_end}


{title:Author}

{pstd}   
{cmd:Yujun,Lian (Arlion)} Department of Finance, Lingnan College, Sun Yat-Sen University, China.{break}
E-mail: {browse "mailto:arlionn@163.com":arlionn@163.com}. {break}
Blog: {browse "http://blog.cnfol.com/arlion":http://blog.cnfol.com/arlion}. {break}
Homepage: {browse "http://www.lingnan.sysu.edu.cn/lnshizi/faculty_vch.asp?name=lianyj":http://www.lingnan.sysu.edu.cn/lnshizi/faculty_vch.asp?name=lianyj}. {break}
MyCourse: {browse "http://www.peixun.net/author/3.html":http://www.peixun.net/author/3.html}. {break} 
Youku Videos: {browse "http://i.youku.com/arlion":http://i.youku.com/arlion}. {break}
{p_end}


{title:Also see}

{pstd} Online: {manhelp egen R};  {help hhi}  {p_end}

{pstd}   {p_end}

