Integrate from 3 StdDevs down instead of starting at 0

src/analysis.rs

@@ -376,11 +376,19 @@ impl Analyzer for NormalAnalyzer {
             if mvn.is_ok() {
                 let mvn = mvn.unwrap();
 
+                // Start 3 standard deviations below the mean, based on
+                // 68-95-99.7 rule, assuming the confidence will be high
+                // enough that 99.7 is close enough to "the whole
+                // distribution" to be reasonable
+                let bip_start = (bridge_ips_mean - (3.0 * bridge_ips_f64.std_dev()).ceil()) as i32;
+                let pr_start =
+                    (positive_reports_mean - (3.0 * positive_reports_f64.std_dev()).ceil()) as i32;
+
                 // Estimate the CDF by integrating the PDF by hand with step
                 // size 1
                 let mut cdf = 0.0;
-                for bip in 0..bridge_ips_today {
-                    for pr in 0..positive_reports_today {
+                for bip in bip_start..bridge_ips_today as i32 {
+                    for pr in pr_start..positive_reports_today as i32 {
                         cdf += mvn.pdf(&DVector::from_vec(vec![bip as f64, pr as f64]));
                     }
                 }