fix:Fix the random prop_sampleM test failure (#186)

by using a 99.99% confidence interval on a Bernoulli distribution.
Since quickCheck already runs 100 tests, this aims for 1/10000,
which corresponds to z-score of 3.89.

Should fix the random CI failure in the prop_sampleM test.

Author: Eiko-Tokura

tests/Monad.hs

@@ -22,6 +22,8 @@ prop_sampleM df = monadic' $ do
     let finalRowCount = D.nRows finalDf
     let realRate = roundToTwoPlaces $ fromIntegral finalRowCount / fromIntegral rowCount
     let diff = abs $ expectedRate - realRate
-    assert (diff <= 0.11)
+    -- calculates the 99.99% confidence interval (quickcheck runs 100 tests, aim for 1/10000)
+    let tolerance = 3.89 * sqrt (expectedRate * (1 - expectedRate) / fromIntegral rowCount)
+    assert (diff <= tolerance)
 
 tests = [prop_sampleM]