macro
Finv cdf a b F

#This macro finds the F value from the F distribution with
#a numerator and b denominator degrees of freedom with
#specified right tail area cdf. Note that Minitab reports the
#left tail area cdf.

#Copyright Mathews and Malnar 12Nov01 All Rights Reserved

#Example calling statement:
#%finv 0.05 4 12 k1
#Answer is k1 = 3.2593

mconstant cdf a b F F1 F2 F3 cdf1 cdf2 cdf3 delta

let F1=0			#create an initial look-up table to begin interpolation
let cdf1=1
let F2=4
let F3 = 80
let cdf3=0
let delta=F3-F1

note Running macro ...

brief 0
while delta>0.001
   %Fcdf F2 a b cdf2
   if cdf>cdf2
      let F3=F2
      let cdf3=cdf2
      let F2=(F1+F2)/2		#binary search algorithm
   else			#cdf<cdf2
      let F1=F2
      let cdf1=cdf2
      let F2=(F2+F3)/2
   endif
   let delta=abs(F3-F1)
endwhile
brief

let F=F2
print F cdf2

endmacro