Spearman-Brown Prophecy

Problem

Return the Spearman-Brown prophecy value k*r / (1 + (k-1)*r) for a test lengthened k times, where r is the current reliability. Return 0.0 when r is not in (-1, 1) or k < 1.

Hint

Reliability rises with test length but with diminishing returns.

Test cases

case 1

in: [0.5,2]

expected: 0.6666666666666666

case 2

in: [0.6,2]

expected: 0.7499999999999999

case 3

in: [0.4,3]

expected: 0.6666666666666667

case 4

in: [0.5,1]

expected: 0.5

case 5

in: [-2,2]

expected: 0

Starter codePython

def spearman_brown(r, k):
    # Your code here
    pass

5 test cases · run your code against them in the editor