# Description:
# Computes the tail probability for the T statistic.
#
clear
# grid of values for T statistic
t = seq(100) / 10.                      
# axis for T statistic
gxaxis("linear", 0, 10, 10, 5)
# axis for tail probability
gyaxis("linear", 0, .2, 2, 10)
# degrees of freedom
df = [1, 2, 4, 7]
# titles
gxtitle("T")
gytitle("probability")
for i = 1 to coldim(df) begin
	# degrees of freedom
	d = fill(df(i), 100, 1) 
	# probability
	p = ttail(t, d)         
	# plot probability as a function of t
	gplot(t, p)             
	# derivative of p with respect to t
	dpdt = coldiff(p) / coldiff(t)
	# points where dpdt >=  -.1 / 10.
	flag = dpdt >= -.1 / 10.
	# determine corresponding indices in t and p
	index = seq(99) + 1
	index = index.row(flag)
	# location of first point where dpdt >= -.1 / 10.
	coord = gcoord(t(index(1)), p(index(1)))
	# label curve with degrees of freedom
	text = ntoa(df(i))
	gaddtext(text, coord, [.5, .5]);
end