HP 17BII+: Update to Normal CDF Formula - Don Phillips
On March 31, 2019, I posted a formula calculate the normal cumulative distributive function (normal CDF) for x ≥ 0:
http://edspi31415.blogspot.com/2019/03/hp-17bii-normal-distribution-and-random.html
Don Phillips provided us with an updated formula to include negative values of x. Much gratitude for allowing me to post this formula:
NCDF=L(ANS:1-EXP(-ABS(X)^2÷2)÷SQRT(2×PI)×(.4361836×L(T:INV(1+.33267×ABS(X)))-.1201676×G(T)^2+.9372980×G(T)^3))×0+IF(X<0:1-G(ANS):G(ANS))
Example Calculations:
X = -2.50, NCDF = 0.00622
X = -1.50, NCDF = 0.06680
X = -0.50, NCDF = 0.30855
X = 0.00, NCDF = 0.50000
X = 0.50, NCDF = 0.69145
X = 1.00, NCDF = 0.84135
X = 2.00, NCDF = 0.97724
Thank you Don, Eddie.
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