def bdtri(k, n, y):
"""Inverse of binomial distribution.
Finds binomial p such that sum of terms 0-k reaches cum probability y.
"""
y = fix_rounding_error(y)
if y < 0.0 or y > 1.0:
raise ZeroDivisionError, "y must be between 1 and 0."
if k < 0 or n <= k:
raise ZeroDivisionError, "k must be between 0 and n"
dn = n - k
if k == 0:
if y > 0.8:
p = -expm1(log1p(y-1.0) / dn)
else:
p = 1.0 - y**(1.0/dn)
else:
dk = k + 1;
p = incbet(dn, dk, 0.5)
if p > 0.5:
p = incbi(dk, dn, 1.0-y)
else:
p = 1.0 - incbi(dn, dk, y)
return p
def bdtrc(k, n, p):
"""Complement of binomial distribution, k+1 through n.
Uses formula bdtrc(k, n, p) = betai(k+1, n-k, p)
See Cephes docs for details.
"""
p = fix_rounding_error(p)
if (p < 0) or (p > 1):
raise ValueError, "Binomial p must be between 0 and 1."
if (k < 0) or (n < k):
raise ValueError, "Binomial k must be between 0 and n."
if k == n:
return 0
dn = n - k
if k == 0:
if p < .01:
dk = -expm1(dn * log1p(-p))
else:
dk = 1 - pow(1.0-p, dn)
else:
dk = k + 1
dk = betai(dk, dn, p)
return dk