Fix issue where vital signs were sometimes being set to NaN

src/main/java/org/mitre/synthea/world/concepts/GrowthChartEntry.java

@@ -6,6 +6,16 @@
  * Represents the LMS parameters on the CDC growth chart for a particular sex and age in months.
  */
 public class GrowthChartEntry implements Serializable {
+
+  /**
+   * The highest percentile to use when attempting to calculate a value. Tests with the BMI growth
+   * charts for females at 240 months show that values greater than 0.99739 cause a term in the
+   * equation to go negative causing the code to return NaN. Values slightly less than that can
+   * return unrealistic values. This provides a limit to the maximum percentile that will be looked
+   * up.
+   */
+  public static double MAX_PERCENTILE = 0.995;
+
   private double lboxCox;
   private double median;
   private double scov;
@@ -51,11 +61,16 @@ public double percentileForValue(double value) {
    * @return The value for the given percentile
    */
   public double lookUp(double percentile) {
-    double z = GrowthChart.calculateZScore(percentile);
+    double percentileToUse = percentile;
+    if (percentile > MAX_PERCENTILE) {
+      percentileToUse = MAX_PERCENTILE;
+    }
+    double z = GrowthChart.calculateZScore(percentileToUse);
     if (this.lboxCox == 0) {
       return this.median * Math.exp((this.scov * z));
     } else {
-      return this.median * Math.pow((1 + (this.lboxCox * this.scov * z)), (1.0 / this.lboxCox));
+      double expressionTerm = this.lboxCox * this.scov * z;
+      return this.median * Math.pow((1 + expressionTerm), (1.0 / this.lboxCox));
     }
   }
 }

src/test/java/org/mitre/synthea/modules/GrowthChartTest.java

@@ -58,7 +58,8 @@ public void testGrowthChartLookupHigh() throws Exception {
   @Test
   public void testGrowthChartLookupMax() throws Exception {
     double height = LifecycleModule.lookupGrowthChart("height", "M", 20, 1.0);
-    assertEquals(94.95447906, height, 0.01);
+    // Max value is now capped to the 99.5th percentile.
+    assertEquals(93.1240785326071, height, 0.01);
   }
 
   @Test

src/test/java/org/mitre/synthea/world/concepts/GrowthChartTest.java

@@ -0,0 +1,20 @@
+package org.mitre.synthea.world.concepts;
+
+import java.util.Map;
+
+import junit.framework.TestCase;
+
+public class GrowthChartTest extends TestCase {
+
+  /**
+   * Test the look up of BMI values at the high end of the range.
+   */
+  public void testLookUp() {
+    Map<GrowthChart.ChartType, GrowthChart> growthChart =
+            GrowthChart.loadCharts();
+    double bmi = growthChart.get(GrowthChart.ChartType.BMI).lookUp(240, "F", 0.997394309960757);
+    assertFalse(Double.isNaN(bmi));
+    bmi = growthChart.get(GrowthChart.ChartType.BMI).lookUp(240, "F", 0.5);
+    assertEquals(21.71699934, bmi, 0.01);
+  }
+}