def _credible_interval(pmf, percentage):
"""
Computes a credible interval for a given distribution.
If percentage=90, computes the 90% CI.
Args:
pmf: Pmf object representing a posterior distribution
percentage: float between 0 and 100
Returns:
sequence of two floats, low and high
"""
cdf = _13_Cdf._make_cdf_from_dict(pmf._get_dict())
prob = (1 - percentage / 100.0) / 2
interval = [cdf._value(p) for p in [prob, 1 - prob]]
return interval
