{smcl}
{* version 1.0.7 27nov2011}{...}
{cmd:help chm}
{hline}

{title:Title}

{p 5}
{cmd:chm} {hline 2} Copy hard missings


{title:Syntax}

{p 5} Use as prefix

{p 8}
{cmd:chm} {varlist} [{cmd:, }{it:options}] {cmd::} {it:command}


{p 5}
where {it:command} is one of 

{p 8}
{help generate} {dtype} {newvar}[{cmd::}{it:lblname}] ={it:{help exp}} 
{ifin}

{p 8}
{help egen} {dtype} {newvar} = {it:fcn}({it:arguments}) {ifin} 
[{cmd:,} {it:egen_options}]

{p 8}
{help replace} {it:oldvar} ={it:{help exp}} {ifin} [{cmd:,} 
{opt nop:romote}]

{p 8}
{help tabulate} {varname} {ifin} {cmd:,} {opt g:enerate}
{cmd:(}{it:stubname}{cmd:)} [{it:tabulate_options}]

{p 8 8}
(if installed){break}
{help center} {varlist} {ifin} {weight} [{cmd:,}{it: center_options}]


{p 5}
Use as command

{p 8}
{cmd:chm} {varlist} {it:oldvar} {ifin} {cmd:,} {opt replace} 
[{it:options}]


{p 5}
where {it:oldvar} is the variable to be replaced


{synoptset 21 tabbed}{...}
{synopthdr}
{synoptline}
{synopt:{opt mvc(mvc)}}replace values in 
{it:newvar}/{it:oldvar}/{it:stubname} with {it:mvc}{p_end}
{synopt:{opt soft}}replace {it:newvar}/{it:oldvar}/{it:stubname} if 
{it:varname} has soft missings (.){p_end}
{synopt:{opt replace}}replace values in {it:oldvar}; required option 
if {cmd:chm} is used as command{p_end}
{synopt:{opt mi}}replace multiply imputed values with missings in 
{it:m}>0{p_end}
{synoptline}
{p 4}
{helpb by} is allowed, if {it:command} allows {bf:by}.

{p 4 8}
Syntax:{break}
{cmd:by} {it:varlist} [, {it:by_options}] {cmd::} {cmd:chm} 
{it:varlist} [, {it:chm_options}] {cmd::} {it:command}


{title:Description}

{pstd}
{cmd:chm} copies hard missings (.a, ..., .z) from variables specified in {it:varlist} to 
{it:newvar}/{it:oldvar}/{it:stubname}. Existing hard missings in {it:newvar} ({it:oldvar} or 
{it:stubname}, respectively) are not replaced, meaning that if the second variable in 
{it:varlist} has hard missings, where the first also has hard missings, hard missing values 
are copied from the latter.

{pstd}
{bf:Remarks:} {cmd:chm} is useful if multiple imputation is planned. In {help mi} 
environment, extended missing values (.a, ..., .z) are viewed as "hard missing" and are not 
imputed. 


{title:Options}

{phang}
{opt mvc(mvc)} replaces values in {it:newvar} ({it:oldvar} or {it:stubname}) with {it:mvc}, 
where {it:mvc} is one of .a, ..., .z.

{phang}
{opt soft} additionally replaces values in {it:newvar} ({it:oldvar} or {it:stubname}) with 
soft missings (.), where variables in {it:varlist} have soft missings. Hard missings in 
{it:newvar}/{it:oldvar}/{it:stubname} are not replaced by soft missings. 
If specified with {opt mvc(mvc)}, {it:mvc} is used to replace values.

{phang}
{opt replace} is required if {cmd:chm} is used as command, because existing 
variables are altered. It is not allowed if {cmd:chm} is used as a prefix.

{phang}
{opt mi} copies hard missings from variables in the original dataset 
after multiple imputation. Each variable in {it:varlist} that has (hard) 
missing values in {it:m}=0, will be replaced with hard missing values in 
{it:m}>0. This option might be useful if imputations are done using 
{help ice} (Royston 2005), since all missing values 
(i.e. {bf:.}, {bf:.a}, ..., {bf:.z}) will be imputed. The data must be in 
{cmd:ice} format, or {help mi_styles:{it:flong style}}. The option may not 
be specified if {cmd:chm} is used as a prefix.


{title:Examples}

{pstd}
{ul:Use as prefix}

	. list

	     +------+
	     | var1 |
	     |------|
	  1. |    1 |
	  2. |   .a |
	  3. |    3 |
	  4. |    . |
	  5. |    5 |
	     +------+

	{cmd:. chm var1 : generate newvar = var1 * 10}

	. list

	     +---------------+
	     | var1   newvar |
	     |---------------|
	  1. |    1       10 |
	  2. |   .a       .a |
	  3. |    3       30 |
	  4. |    .        . |
	  5. |    5       50 |
	     +---------------+

	{cmd:. chm var1 ,mvc(.m) : tabulate var1 ,generate(stubname)}

	. list var1 stubname?

	     +---------------------------------------+
	     | var1   stubna~1   stubna~2   stubna~3 |
	     |---------------------------------------|
	  1. |    1          1          0          0 |
	  2. |   .a         .m         .m         .m |
	  3. |    3          0          1          0 |
	  4. |    .          .          .          . |
	  5. |    5          0          0          1 |
	     +---------------------------------------+

	{cmd:. chm var1 : replace var1 = 1}

	. list var1

	     +------+
	     | var1 |
	     |------|
	  1. |    1 |
	  2. |   .a |
	  3. |    1 |
	  4. |    1 |
	  5. |    1 |
	     +------+

{pstd}
{ul:Use as command}

	{cmd:. chm newvar var1 ,replace soft}

	. list var1 newvar

	     +---------------+
	     | var1   newvar |
	     |---------------|
	  1. |    1       10 |
	  2. |   .a       .a |
	  3. |    1       30 |
	  4. |    .        . |
	  5. |    1       50 |
	     +---------------+


{title:References}

{pstd}
Royston, P. (2005) Multiple imputation of missing values: update. Stata 
Journal 5 (2), 188-201.

		 
{title:Author}

{pstd}Daniel Klein, University of Bamberg, klein.daniel.81@gmail.com


{title:Also see}

{psee}
Online: {helpb replace}, {help generate}, {help egen}, {help tabulate}, {help mi}{p_end}
{psee}
if installed: {help center}, {help ice}{p_end}