program define stars
    version 4.0
    
    global D_sm11 "   This lab illustrates how one can use the Chi-square goodness of fit test to test whether a random sample comes from a particular distribuiton. You pick a sample size (n) and a number of bins (bins) and the lab generates n points that fall in the square of side 1 whose lower left corner is the origin."
    wdctl static D_sm11 5 5 290 32

    global D_sm12 "   The lab tests whether the points are randomly placed in the square by placing a (bins by bins) grid on the square, counting how many points fall in each little square, and using the Chi-square statistic to compare the observed counts to the expected counts. The lab displays the value of the statistic and its p-value."
    wdctl static D_sm12 5 40 290 40

*    global D_sm13 "An Example: Let X be the number of times we flip a coin until we get the 5th head. This would be Negative Binomial with n=5 and pi=.5."
*    wdctl static D_sm13 5 85 290 16

    global D_sm14 "Notes: 1) The expected number of points in each square is n/(bins^2). For the Chi-square statistic to be valid, this number must be at least 5. If you choose an n and bins for which this is not true, you will get an error message.   2) The degrees of freedom of the Chi-square statistic is the number of squares, that is, bins^2.   3) You can use the Tests of Significance Lab to verify that the p-value is correct."
    wdctl static D_sm14 75 85 200 60

    global D_sm6 "n"
    global D_sm7 "10 50 100 200 300 400 500"
    wdctl static D_sm6         5 85 10 10
    global D_var1 "500"
    wdctl ssimple D_var1 D_sm7 5 95 25 70

    global D_sm8 "bins"
    global D_sm9 "2 3 4 5 6 7 8 9 10"
    wdctl static D_sm8         40 85 20 10
    global D_var2 "5"
    wdctl ssimple D_var2 D_sm9 40 95 25 85

    wdctl button "Run"     115 160 30 14 D_b1
    wdctl button "Close"   150 160 30 14 D_b2
    wdctl button "Help"    185 160 30 14 D_b3 

    global D_b1 "starsdr"
    global D_b2 "exit 1234"
    global D_b3 "whelp stars"

    cap noi wdlg "Chi-square Goodness of Fit Test" $D_dlgx $D_dlgy 300 240

end

program define starsdr
    version 4.0

    preserve
    clear

    local n=$D_var1
    local nb=$D_var2

    local nb2=`nb'*`nb'

    if `n'/`nb2' < 5 {
        sstopbox stop "n/(bins^2) must be at least 5"
        exit}

    gph open
    gph pen 1

    set obs `n'
    gen x=uniform()
    gen y=uniform()
    graph y x, gap(1) s(i) xlab(0,.25,.50,.75,1) ylab(0,.25,.50,.75,1) bbox(3000,000,19000,16000,850,400,0)  l1(" ") l2(" ") b1(" ") b2(" ")
    gphconv

    replace x=uniform()
    replace y=uniform()
    points2 x y 50 1

    local nbp1=`nb'+1

    gen x1=0 in 1/`nbp1'
    gen x2=1 in 1/`nbp1'
    gen x3=(_n-1)/`nb'

    segments x3 x1 x3 x2
    segments x1 x3 x2 x3

    
    graph y x, gap(1) s(i) xlab(0,.25,.50,.75,1) ylab(0,.25,.50,.75,1) bbox(3000,15000,19000,31000,850,400,0)  l1(" ") l2(" ") b1(" ") b2(" ")
    gphconv    
    segments x3 x1 x3 x2
    segments x1 x3 x2 x3


    gen bin=int(`nb'*y)*`nb'+int(`nb'*x)+1
    sort bin
    by bin: gen int cx=_N if _n==1
    sort cx
*    replace cx=0 if cx==.

    replace x1=.
    replace x2=.

    replace x1=(mod(bin-1,`nb')/`nb')+(1/(2*`nb')) 
    replace x2=(int((bin-1)/`nb')/`nb')+(1/(2*`nb'))
    gen str3 z=string(int(cx)) if cx!=.
    texts x1 x2 z

    local expn=`n'/(`nb'*`nb')
    replace cx=(cx-`expn')^2/`expn'
    summarize cx
    local chi2: display %5.2f _result(1)*_result(3)
    local df=`nb'*`nb'
    local pv: display %5.2f chiprob(`df',`chi2')
    gph text 1000 16000 0 0 Chi-square=`chi2', df=`df', p-value=`pv'
    gph text 2000 16000 0 0 n=`n', Expected in each cell=`expn'
   
    gph close

    restore

end
exit