*! version 5.0 2Nov1997
* Written by Jonah B. Gelbach, Department of Economics, University of Maryland

************************************************************************************
* 
* This program calculates all cell means, differences, and differences
* in differences needed to create difs-in-difs tables.
* The syntax is as follows:
* 
* . did varlist [if] , rows( row-variable ) columns( column-variable )
*
* where row-variable is the variable allowed to change across the two
* rows, column-variable is the variable allowed to change across the two
* columns. varlist may have only one entry.
************************************************************************************


program define did2

local varlist "required existing max(1)"
local if "optional"

**** old options line ****
**local options "ROWS(string) COLUMNS(string) Table(integer 1)  Start(integer 1) ROWDIFS(string)"**
**** old options line ****

local options "ROWS(string) COLUMNS(string) ROWDIFS(string)"
parse "`*'"

parse "`varlist'", parse(" ")
local number = 0
local colcoun =  `start' - 1
local depvar = substr("`1'", 1, 6)

if "`rowdifs'" == "" { local rowdifs "rowdif" }

tempvar use
quietly mark `use' `if' `in'
quietly markout `use' `varlist' `rows' `columns'


if "`if'" ~= ""	{ local if "`if' & `use' == 1 "	}
	else	{ local if "if `use' == 1 "	}

local oldif "`if'"

quietly summarize `rows' `if' 
local rowmin = _result(5)
local rowmax = _result(6)

quietly summarize `columns' `if'
local colmin = _result(5)  		/* getting min value of columns variable*  */
local colmax = _result(6)		/* getting max value of columns variable*  */

local rows2 = "`rowmin'"
local rows1 = "`rowmax'"

tempvar rowvar0 colvar0 rowvar1 colvar1 inter
gen `rowvar1' = `rows' == `rowmax'	/* making dummy for row variable equals its max */
gen `colvar1' = `columns' == `colmax'	/* making dummy for col variable equals its max */
*gen `inter' = `rowvar1'*`colvar1'	/* interacting row and column dummies */

gen `rowvar0' = `rowvar1' == 0		/* making dummy for row variable equals its min */
gen `colvar0' = `colvar1' == 0		/* making dummy for col variable equals its min */


if "`if'" ~= ""	{ local if "`if' & " 	}
	else	{ local if "if"		}

*di "`if'"

quietly regress `varlist' `rowvar1' `rowvar0' `if' `colvar1' == 1, nocons	/* holding colvar = max */
local numobs = _result(1)
tempname b v
matrix `b' = get(_b)
matrix `v' = get(VCE)
local b11 = `b'[1,1]		/* Mean for row max, col max */
local b21 = `b'[1,2]		/* Mean for row min, col max */
local s11 = sqrt(`v'[1,1])	/* SE for row max, col max */
local s21 = sqrt(`v'[2,2])	/* SE for row min, col max */


quietly regress `varlist' `rowvar1' `rowvar0' `if' `colvar0', nocons	/* holding colvar = min */
local numobs = `numobs' + _result(1)
matrix `b' = get(_b)
matrix `v' = get(VCE)
local b12 = `b'[1,1]		/* Mean for row max, col min */
local b22 = `b'[1,2]		/* Mean for row min, col min */
local s12 = sqrt(`v'[1,1])	/* SE for row max, col min */
local s22 = sqrt(`v'[2,2])	/* SE for row min, col min */

local b13 = `b11' - `b12'	/* Differencing across column var, holding row at max */
local b23 = `b21' - `b22'	/* Differencing across column var, holding row at min */
local b31 = `b11' - `b21'	/* Differencing across row var, holding column at max */
local b32 = `b12' - `b22'	/* Differencing across row var, holding column at min */

local b33 = `b31' - `b32'	/* Difference in Differences Estimate */

local s13 = sqrt(`s11'^(2) + `s12'^(2))	/* SE for dif across column var, holding row at max */
local s23 = sqrt(`s21'^(2) + `s22'^(2))	/* SE for dif across column var, holding row at min */
local s31 = sqrt(`s11'^(2) + `s21'^(2))	/* SE for dif across row var, holding column at max */
local s32 = sqrt(`s12'^(2) + `s22'^(2))	/* SE for dif across row var, holding column at min */
			         
local s33 = sqrt(`s31'^(2) + `s32'^(2))	/* SE for Difference in Differences Estimate */



display _newline _newline
display "The difs-in-difs estimates will be computed with `numobs' observations."
local i 1 


********************************************************
********	Making Viewer-Friendly Table	********
********************************************************

local r 1
local c 1
while `r' < 4 {
  while `c' < 4 {
		global s`r'`c' "`s`r'`c''"
		global b`r'`c' "`b`r'`c''"
		local b`r'`c' : display %9.3f `b`r'`c''
		local s`r'`c' : display %9.3f `s`r'`c''
		parse "`s`r'`c''", parse(" ")
		local s`r'`c' = "`1'"
		local c = `c' + 1
					}	/* ending columns loop */
		local c 0
		local r = `r' + 1
					 }	/* ending rows loop */

di "tab `rows' `cols' `oldif'"
tab `rows' `columns' `oldif'

di _newline _newline 

di "Results for Dependent Variable *`varlist'*: " _newline

di "                         C o l u m n    V a r i a b l e"
di "                -------------------------------------------"
di _column(17) "`columns' = `colmax' " _column(33)	"`columns' = `colmin'"  _column(49) "Differences"

di "Row Variable:"
di "-------------"
di "`rows' = " _column(13) "`rowmax'" _column(17) `b11' _column(33) `b12' _column(49) `b13' 
di _column(17) "(`s11')" _column(33) "(`s12')" _column(49) "(`s13')" _newline

di "`rows' = " _column(13) "`rowmin'" _column(17) `b21' _column(33) `b22' _column(49) `b23'
di _column(17) "(`s21')" _column(33) "(`s22')" _column(49) "(`s23')" _newline

di "Row Diff " _column(17) `b31' _column(33) `b32' _column(49) `b33' 
di _column(17) "(`s31')" _column(33) "(`s32')" _column(49) "(`s33')" _newline



end



*
*old stuff for my purposes:
*
*
**************************************************************************************
**** Beginning routine for first column (relative to textable) of current variable ***
**************************************************************************************
*
*local colcoun = 1 + `colcoun'
*display _newline _newline
*
*
*display ". *table begin `table' `colcoun' heading= `columns'=`colmax'" _newline 
*display "Holding column variable constant at `columns' == `colmax'" _newline
*display "Row difference for column variable `columns' == `colmax'  follows" _newline
*hyphens, lines(3)
*display "    Mean |   Estimate    Std. Err.       "
*display "---------+--------------------------------------------------------------------"
*while `i' <= 2	{
*  display "`depvar'`i'" _column(10) "| "  `b`i'1' _column(25)  `s`i'1' " . . . . `rows' == `rows`i''"
*  local i = `i' + 1
*		}
*
*display "`rowdifs'" _column(10) "| " `b31' _column(25) `s31' " . . . . Difference" 
*display "------------------------------------------------------------------------------" _newline
*display ". *table end"
*
*
***************************************************************************************
**** Beginning routine for second column (relative to textable) of current variable ***
***************************************************************************************
*
*local i 1
*local colcoun = 1 + `colcoun'
*display _newline _newline
*display ". *table begin `table' `colcoun' heading= `columns'=`colmin'" _newline
*display "Holding column variable constant at `columns' == `colmin'"  _newline
*display "Row difference for column variable `columns' == `colmin'  follows" _newline
*hyphens, lines(3)
*display "    Mean |   Estimate    Std. Err.       "
*display "---------+--------------------------------------------------------------------"
*while `i' <= 2	{
*  display "`depvar'`i'"  _column(10) "| "  `b`i'2' _column(25)  `s`i'2' " . . . . `rows' == `rows`i''""
*  local i = `i' + 1
*		}
*
*display "`rowdifs'" _column(10) "| " `b32' _column(25) `s32' " . . . . Difference" 
*display "------------------------------------------------------------------------------" _newline
*display ". *table end"
*
*
**************************************************************************************
**** Beginning routine for third column (relative to textable) of current variable ***
**************************************************************************************
*
*local i 1
*local colcoun = 1 + `colcoun'
*display _newline _newline _newline
*display ". *table begin `table' `colcoun' heading= (1)-(2)" _newline
*display "Differences across columns:" _newline
*hyphens, lines(3)
*display "    Mean |   Estimate    Std. Err.       "
*display "---------+--------------------------------------------------------------------"
*while `i' <= 2	{
*  display "`depvar'`i'"  _column(10) "| "  `b`i'3' _column(25)  `s`i'3' " . . . . `rows' == `rows`i''""
*  local i = `i' + 1
*		}
*
*display "`rowdifs'" _column(10) "| " `b33' _column(25) `s33' " . . . . Dif-in-difs" 
*display "------------------------------------------------------------------------------" _newline
*display ". *table end"
*