program define t4
    version 4.0

    preserve
    clear
    gph open
    gph pen 1

    set obs 501

    gen x=6*(uniform()-.5)
    gen y=.4*uniform()
    graph y x,xlab(-3,-2,-1,0,1,2,3) ylab(0,.1,.2,.3,.4) s(i)
    gphconv

    replace x=6*((_n-251)/500)

    gph pen 2
    pdf_Z x y
    lines x y

    gph pen 1
    pdf_t x y 3
    lines x y

    pdf_t x y 5
    lines x y

    pdf_t x y 10
    lines x y

    pdf_t x y 20
    lines x y

    gph text 1000 16000 0 0 Z curve and t curves for 3, 5, 10, and 20 degrees of freedom

    gph close

    restore
exit
end