from math import sqrt
def correlation_coefficient(word):
alphabet = 'abcdefghijklmnopqrstuvwxyz'
x = list(range(len(word)))
y = list(map(alphabet.index, word))
n = len(word)
top = n * sum(a*b for a, b in zip(x, y)) - sum(x) * sum(y)
bottom = (sqrt(n * sum(a ** 2 for a in x) - sum(x) ** 2)
* sqrt(n * sum(a ** 2 for a in y) - sum(y) ** 2))
r = top / bottom
return r
print(correlation_coefficient('forty')) # => 0.9644068036349517
print(correlation_coefficient('bells')) # => 0.9717931994369533
print(correlation_coefficient('correlation')) # => 0.13063589827847785
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