Index: Ringtoets/GrassCoverErosionInwards/src/Ringtoets.GrassCoverErosionInwards.Service/GrassCoverErosionInwardsOutputCalculationService.cs
===================================================================
diff -u -r3739d94e5ef665dc97d80c8c3f7bc6ef4b19d93c -r89e5c41b732ccf30350026e8d35a01d9a180fdfc
--- Ringtoets/GrassCoverErosionInwards/src/Ringtoets.GrassCoverErosionInwards.Service/GrassCoverErosionInwardsOutputCalculationService.cs	(.../GrassCoverErosionInwardsOutputCalculationService.cs)	(revision 3739d94e5ef665dc97d80c8c3f7bc6ef4b19d93c)
+++ Ringtoets/GrassCoverErosionInwards/src/Ringtoets.GrassCoverErosionInwards.Service/GrassCoverErosionInwardsOutputCalculationService.cs	(.../GrassCoverErosionInwardsOutputCalculationService.cs)	(revision 89e5c41b732ccf30350026e8d35a01d9a180fdfc)
@@ -52,7 +52,7 @@
         }
 
         /// <summary>
-        /// Calculates the <see cref="ProbabilisticOutput"/> given the <paramref name="calculation"/>, and <paramref name="reliability"/>.
+        /// Calculates the <see cref="ProbabilisticOutput"/> given the <paramref name="calculation"/> and <paramref name="reliability"/>.
         /// </summary>
         /// <param name="calculation">The calculation which is used.</param>
         /// <param name="reliability">The reliability result.</param>
@@ -80,14 +80,14 @@
 
         private void Calculate()
         {
-            CalculateReliability();
+            CalculateProbability();
 
             CalculateRequiredReliability();
 
             factorOfSafety = FactorOfSafety(reliability, requiredReliability);
         }
 
-        private void CalculateReliability()
+        private void CalculateProbability()
         {
             requiredProbability = RequiredProbability(contribution, norm, lengthEffectN);
             probability = ReliabilityToProbability(reliability);
@@ -100,19 +100,19 @@
 
         #region Sub calculations
 
-        private static double RequiredProbability(double probability, double contribution, double lengthEffectN)
+        private static double RequiredProbability(double contribution, double norm, double lengthEffectN)
         {
-            return probability*(1/contribution)/lengthEffectN;
+            return contribution*(1/norm)/lengthEffectN;
         }
 
         private static double ReliabilityToProbability(double reliability)
         {
             return Normal.CDF(0, 1, -reliability);
         }
 
-        private static double ProbabilityToReliability(double requiredProbability)
+        private static double ProbabilityToReliability(double probability)
         {
-            return Normal.InvCDF(0, 1, 1 - requiredProbability);
+            return Normal.InvCDF(0, 1, 1 - probability);
         }
 
         private static double FactorOfSafety(double reliability, double requiredReliability)
Index: Ringtoets/GrassCoverErosionInwards/test/Ringtoets.GrassCoverErosionInwards.Data.Test/GrassCoverErosionInwardsFailureMechanismTest.cs
===================================================================
diff -u -r4936ea40e490dd8a3ed500e1c5a8f8390ff31491 -r89e5c41b732ccf30350026e8d35a01d9a180fdfc
--- Ringtoets/GrassCoverErosionInwards/test/Ringtoets.GrassCoverErosionInwards.Data.Test/GrassCoverErosionInwardsFailureMechanismTest.cs	(.../GrassCoverErosionInwardsFailureMechanismTest.cs)	(revision 4936ea40e490dd8a3ed500e1c5a8f8390ff31491)
+++ Ringtoets/GrassCoverErosionInwards/test/Ringtoets.GrassCoverErosionInwards.Data.Test/GrassCoverErosionInwardsFailureMechanismTest.cs	(.../GrassCoverErosionInwardsFailureMechanismTest.cs)	(revision 89e5c41b732ccf30350026e8d35a01d9a180fdfc)
@@ -124,5 +124,20 @@
             Assert.IsTrue(calculations.All(c => c is GrassCoverErosionInwardsCalculation));
             mocks.VerifyAll();
         }
+
+        [Test]
+        public void Contribution_Always_UpdatesNormProbabilityInputContribution()
+        {
+            // Setup
+            var failureMechanism = new GrassCoverErosionInwardsFailureMechanism();
+            const double newContribution = 12;
+
+            // Call
+            failureMechanism.Contribution = newContribution;
+
+            // Assert
+            Assert.AreEqual(newContribution, failureMechanism.NormProbabilityInput.Contribution);
+            Assert.AreEqual(newContribution, failureMechanism.Contribution);
+        }
     }
 }
\ No newline at end of file
Index: Ringtoets/HeightStructures/src/Ringtoets.HeightStructures.Plugin/HeightStructuresGuiPlugin.cs
===================================================================
diff -u -ra61d9a646a09ddee65775c179bba12c40003cf84 -r89e5c41b732ccf30350026e8d35a01d9a180fdfc
--- Ringtoets/HeightStructures/src/Ringtoets.HeightStructures.Plugin/HeightStructuresGuiPlugin.cs	(.../HeightStructuresGuiPlugin.cs)	(revision a61d9a646a09ddee65775c179bba12c40003cf84)
+++ Ringtoets/HeightStructures/src/Ringtoets.HeightStructures.Plugin/HeightStructuresGuiPlugin.cs	(.../HeightStructuresGuiPlugin.cs)	(revision 89e5c41b732ccf30350026e8d35a01d9a180fdfc)
@@ -148,7 +148,7 @@
             {
                 failureMechanism = failureMechanismContext.WrappedData;
             }
-            return failureMechanism != null && ReferenceEquals(view.Data, (failureMechanism).SectionResults);
+            return failureMechanism != null && ReferenceEquals(view.Data, failureMechanism.SectionResults);
         }
 
         #endregion
Index: Ringtoets/HeightStructures/test/Ringtoets.HeightStructures.Data.Test/HeightStructuresFailureMechanismTest.cs
===================================================================
diff -u -r7ddd944b70d252ae493bba48ea9b31c01634082d -r89e5c41b732ccf30350026e8d35a01d9a180fdfc
--- Ringtoets/HeightStructures/test/Ringtoets.HeightStructures.Data.Test/HeightStructuresFailureMechanismTest.cs	(.../HeightStructuresFailureMechanismTest.cs)	(revision 7ddd944b70d252ae493bba48ea9b31c01634082d)
+++ Ringtoets/HeightStructures/test/Ringtoets.HeightStructures.Data.Test/HeightStructuresFailureMechanismTest.cs	(.../HeightStructuresFailureMechanismTest.cs)	(revision 89e5c41b732ccf30350026e8d35a01d9a180fdfc)
@@ -126,5 +126,20 @@
             Assert.IsTrue(calculations.All(c => c is HeightStructuresCalculation));
             mocks.VerifyAll();
         }
+
+        [Test]
+        public void Contribution_Always_UpdatesNormProbabilityInputContribution()
+        {
+            // Setup
+            var failureMechanism = new HeightStructuresFailureMechanism();
+            const double newContribution = 12;
+
+            // Call
+            failureMechanism.Contribution = newContribution;
+
+            // Assert
+            Assert.AreEqual(newContribution, failureMechanism.NormProbabilityInput.Contribution);
+            Assert.AreEqual(newContribution, failureMechanism.Contribution);
+        }
     }
 }
\ No newline at end of file
Index: Ringtoets/Integration/test/Ringtoets.Integration.Data.Test/AssessmentSectionTest.cs
===================================================================
diff -u -r7ddd944b70d252ae493bba48ea9b31c01634082d -r89e5c41b732ccf30350026e8d35a01d9a180fdfc
--- Ringtoets/Integration/test/Ringtoets.Integration.Data.Test/AssessmentSectionTest.cs	(.../AssessmentSectionTest.cs)	(revision 7ddd944b70d252ae493bba48ea9b31c01634082d)
+++ Ringtoets/Integration/test/Ringtoets.Integration.Data.Test/AssessmentSectionTest.cs	(.../AssessmentSectionTest.cs)	(revision 89e5c41b732ccf30350026e8d35a01d9a180fdfc)
@@ -150,6 +150,8 @@
 
             Assert.AreEqual(30000.0, section.HeightStructures.NormProbabilityInput.Norm);
 
+            Assert.AreEqual(30000.0, section.GrassCoverErosionInwards.NormProbabilityInput.Norm);
+
             Assert.AreEqual(sum, section.FailureMechanismContribution.Distribution.Sum(d => d.Contribution));
         }
 
Index: Ringtoets/Piping/src/Ringtoets.Piping.Data/PipingCalculation.cs
===================================================================
diff -u -r0a94ed42cb943659d68be2ce6fb430f8f29fc3f3 -r89e5c41b732ccf30350026e8d35a01d9a180fdfc
--- Ringtoets/Piping/src/Ringtoets.Piping.Data/PipingCalculation.cs	(.../PipingCalculation.cs)	(revision 0a94ed42cb943659d68be2ce6fb430f8f29fc3f3)
+++ Ringtoets/Piping/src/Ringtoets.Piping.Data/PipingCalculation.cs	(.../PipingCalculation.cs)	(revision 89e5c41b732ccf30350026e8d35a01d9a180fdfc)
@@ -42,10 +42,6 @@
         /// <paramref name="normProbabilityInputParameters"/> is <c>null</c>.</exception>
         public PipingCalculation(GeneralPipingInput generalInputParameters, NormProbabilityPipingInput normProbabilityInputParameters)
         {
-            if (generalInputParameters == null)
-            {
-                throw new ArgumentNullException("generalInputParameters");
-            }
             if (normProbabilityInputParameters == null)
             {
                 throw new ArgumentNullException("normProbabilityInputParameters");
Index: Ringtoets/Piping/test/Ringtoets.Piping.Data.Test/PipingFailureMechanismTest.cs
===================================================================
diff -u -r41fac7fff0a505c08945108d795dcb877f10b816 -r89e5c41b732ccf30350026e8d35a01d9a180fdfc
--- Ringtoets/Piping/test/Ringtoets.Piping.Data.Test/PipingFailureMechanismTest.cs	(.../PipingFailureMechanismTest.cs)	(revision 41fac7fff0a505c08945108d795dcb877f10b816)
+++ Ringtoets/Piping/test/Ringtoets.Piping.Data.Test/PipingFailureMechanismTest.cs	(.../PipingFailureMechanismTest.cs)	(revision 89e5c41b732ccf30350026e8d35a01d9a180fdfc)
@@ -218,5 +218,20 @@
             // Assert
             CollectionAssert.DoesNotContain(failureMechanism.CalculationsGroup.Children, folder);
         }
+
+        [Test]
+        public void Contribution_Always_UpdatesNormProbabilityInputContribution()
+        {
+            // Setup
+            var failureMechanism = new PipingFailureMechanism();
+            const double newContribution = 12;
+
+            // Call
+            failureMechanism.Contribution = newContribution;
+
+            // Assert
+            Assert.AreEqual(newContribution, failureMechanism.NormProbabilityInput.Contribution);
+            Assert.AreEqual(newContribution, failureMechanism.Contribution);
+        }
     }
 }
\ No newline at end of file