diff --git a/src/calc.rs b/src/calc.rs
index fea13a8..a63f667 100644
--- a/src/calc.rs
+++ b/src/calc.rs
@@ -27,7 +27,7 @@ pub fn hyper_geometric(
             hyper_geometric_exactly(population_size, successes, sample_size, sample_successes);
         let (less_than, less_or_equal, greater_or_equal, greater_than) =
             if sample_successes < sample_size / 2 {
-                let less_than: f64 = (1..sample_size)
+                let less_than: f64 = (0..sample_successes)
                     .map(|i| hyper_geometric_exactly(population_size, successes, sample_size, i))
                     .sum();
                 let less_or_equal = less_than + exactly;