Binomial probability calculation function in SQL (BigQuery)

binomial_prob.sql

-- Public Domain CC0 license. https://creativecommons.org/publicdomain/zero/1.0/

-- Calculate the probability of k successes for n trials with probability of success k, 
-- using the binomial distribution.

-- Calculate the binomial coefficient using the "multiplicative formula"
CREATE OR REPLACE FUNCTION functions.binomial_coef(n INT64, k INT64) AS ((
  -- k!/(n!*(n-k)!)
  -- We're going to have a hard time doing factorials here,
  -- but based on the "multiplicative formula" in Wiki, it should be possible:
  -- https://en.wikipedia.org/wiki/Binomial_coefficient#Computing_the_value_of_binomial_coefficients
  -- Taken together with the fact that we can do EXP(SUM(LOG(x))) to do product over multiple rows, we have:
  SELECT
    -- Binomial coefficient is always an integer,
    -- but BigQuery is not super happy about EXP(SUM(LN(x))) trick,
    -- so the ROUND is added to be safe :-)
    ROUND(EXP(SUM(LN((n + 1 - i) / i))))
  FROM
    -- Take advantage of symmetry (n // k) == (n // (n-k))
    UNNEST(GENERATE_ARRAY(1, IF(k > n - k, n - k, k), 1)) i
));

-- Calculate the probability of k successes in n trials where the probability of a success is p.
-- This uses the probability density function for the binomial distribution.
-- See example below.
CREATE OR REPLACE FUNCTION functions.binomial_prob(n INT64, k INT64, p FLOAT64) AS ( 
  functions.binomial_coef(n, k) * POW(p, k) * POW(1-p, n-k)
);

-- The example from Wikipedia: https://en.wikipedia.org/wiki/Binomial_distribution#Example
-- Probability of getting 4 heads in 6 tosses with a coin that is biased and has heads probability 0.3
-- Calculates to 0.05953499999999999
SELECT functions.binomial_prob(6, 4, 0.3);

Aha, that makes sense @benman1 I'm happy to hear it worked out for you with a different solution 🙂