NORMALD_ICDF (LEFT/CENTER/RIGHT)

[Wes Loewer]

(02-20-2020 09:03 PM)StephanP Wrote:  NORMALD_ICDF(185,10,.85,RIGHT)=NORMALD_ICDF(185,10,1-0.85)=174.635...
NORMALD_ICDF(185,10,.85,CENTER)={170.604...; 199.395...}

I've been thinking about this suggestion. Using RIGHT or CENTER works well on the TI-84+ CE because there is a dialog box ("Wizard") that makes selecting RIGHT or CENTER very easy. If I had to actually type out "RIGHT" then I think I would resort to using 1-0.85.

Since Prime stats functions already allow for an optional number of arguments, what if calling NORMALD_ICDF with no arguments popped up dialog box, say something like:

Code:

//////////////////////////
// Inverse Normal CDF   //
//////////////////////////

// local but persistent
LOCAL mu=0, sigma=1, area=0.05, tail=1;

EXPORT NormalInv()
BEGIN
 LOCAL status;

 status:=INPUT(
   {
     {mu,[0],{20,50,0}},
     {sigma,[0],{20,50,1}},
     {area,[0],{20,50,2}},
     {tail,{"left","right","inner","outer"},{20,25,3}}
   },
   "Inverse Normal CDF",
   {"Mean:","StdDev:","Area:","Tail:"},
   {"mean of the distribution","standard deviation of the distribution","area (probability)","left, right, inner, or outer area"}
  );
 IF status==0 THEN
  RETURN "canceled";
 END;
 CASE
  // left
  IF tail==1 THEN RETURN NORMALD_ICDF(mu,sigma,area); END
  // right
  IF tail==2 THEN RETURN -NORMALD_ICDF(-mu,sigma,area); END // more precise than NORMALD_ICDF(mu,sigma,1-area) for very small areas
  // inner
  IF tail==3 THEN RETURN {NORMALD_ICDF(mu,sigma,(1-area)/2),-NORMALD_ICDF(-mu,sigma,(1-area)/2)}; END
  // outer (2 tail)
  IF tail==4 THEN RETURN {NORMALD_ICDF(mu,sigma,area/2),-NORMALD_ICDF(-mu,sigma,area/2)}; END
  DEFAULT RETURN "invalid tail";
 END;
END;

When the 84+ introduced the Wizard option for stats functions back in 2011, it make a huge difference in my stats classes. Previously, my students always had difficulty remembering the order of the arguments in the commands. With Stats Wizards, that problem instantly went away.