INVT (Inverse t) Program for TI-83 or TI-83 PLUS

INVT (Inverse t) Program for TI-83 or TI-83 PLUS
Written by Mr. Hansen and the Class of 1999
Rev. 10/17/2000

Purpose: Just as invNorm accepts a quantile value (between 0 and 1, inclusive) and returns a z score, the INVT program accepts a value between 0 and 1, plus user-specified degrees of freedom (df), and returns a t score. INVT thus fills a gap that TI left when designing the firmware of the TI-83.

Simple Program Listing:

:ClrHome
:Disp "INVERSE T"
:Disp "BY EMH, 3-8-1999"
:Disp "CUM. PROB.":Prompt P
:Disp "DF (INTEGER ³ 1)":Prompt D
:D+1®N
:If P<1E^-9 or P>1-1E^-9 or D<1 or D¹round(D,0)
:Then
:Disp "DOMAIN ERROR"
:Stop
:End
:TInterval 0,Ö(N),N,abs(2P-1)
:If P³.5
:Then
:upper®T
:Else
:lower®T
:End
:Output(6,11,"T=")
:T

Listing of longer version with better error checking:

:ClrHome
:Disp "INVERSE T"
:Disp "BY EMH, 10-17-00"
:Disp "CUMULATIVE PROB.":Input P
:If P<1E^-9 or P>1-1E^-9
:Then
:If P£0 or P³1
:Then
:Disp "REMEMBER: HERE,"
:Disp " 0 < PROB. < 1"
:Stop
:End
:If P<1E^-9
:Then
:Disp "T= -INFINITY"
:^-9.999E99®T
:Stop
:End
:If P>1-1E^-9
:Then
:Disp "T= +INFINITY"
:9.999E99®T
:Stop
:End
:End
:Disp "DF (INTEGER ³ 1)":Input D
:D+1®N
:If D<1 or D¹round(D,0)
:Then
:Disp "DF MUST BE AN"
:Disp " INTEGER ³ 1"
:Stop
:End
:TInterval 0,Ö(N),N,abs(2P-1)
:If P³.5
:Then
:upper®T
:Else
:lower®T
:End
:Output(6,11,"T=")
:T

Comments:

Use 2nd CATALOG to find TInterval and any other symbols, such as <, which you may not have used before.
The ® symbol is the STO key, located just above the ON button in the lower left corner of the calculator.
The special variables "upper" and "lower" can be found at the very bottom of the VARS Statistics TEST menu.
Both versions are written so that the Answer variable (2nd Ans) and the variable T are both left storing the result value at the conclusion of the program.