Index: Ringtoets/Common/src/Ringtoets.Common.Data/FailureMechanism/FailureMechanismSectionResult.cs
===================================================================
diff -u -r0b081b2391e73b7e3ff04ef31fafd2df1b1571e3 -r58c0d0a9d882210f47175f62ef1e4c494fce140b
--- Ringtoets/Common/src/Ringtoets.Common.Data/FailureMechanism/FailureMechanismSectionResult.cs	(.../FailureMechanismSectionResult.cs)	(revision 0b081b2391e73b7e3ff04ef31fafd2df1b1571e3)
+++ Ringtoets/Common/src/Ringtoets.Common.Data/FailureMechanism/FailureMechanismSectionResult.cs	(.../FailureMechanismSectionResult.cs)	(revision 58c0d0a9d882210f47175f62ef1e4c494fce140b)
@@ -21,6 +21,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using Core.Common.Base;
 using Core.Common.Base.Data;
 
@@ -67,7 +68,13 @@
         /// <summary>
         /// Gets and sets the value of assessment layer two a.
         /// </summary>
-        public RoundedDouble AssessmentLayerTwoA { get; set; }
+        public RoundedDouble AssessmentLayerTwoA
+        {
+            get
+            {
+                return GetAssessmentResult();
+            }
+        }
 
         /// <summary>
         /// Gets and sets the value of assessment layer two b.
@@ -83,5 +90,18 @@
         /// Gets and sets a list of <see cref="ICalculationScenario"/>
         /// </summary>
         public List<ICalculationScenario> CalculationScenarios { get; private set; }
+
+        private RoundedDouble GetAssessmentResult()
+        {
+            var relevantScenarios = CalculationScenarios.Where(cs => cs.IsRelevant).ToList();
+            double totalContribution = relevantScenarios.Aggregate<ICalculationScenario, double>(0, (current, calculationScenario) => current + calculationScenario.Contribution);
+
+            if (relevantScenarios.Any() && Math.Abs(totalContribution - 1.0) > 1e-6)
+            {
+                return (RoundedDouble) double.NaN;
+            }
+
+            return new RoundedDouble();
+        }
     }
 }
\ No newline at end of file
Index: Ringtoets/Common/src/Ringtoets.Common.Forms/Properties/Resources.Designer.cs
===================================================================
diff -u -r3abef5ccb0ba5386c65d4156b319970fc4337a29 -r58c0d0a9d882210f47175f62ef1e4c494fce140b
--- Ringtoets/Common/src/Ringtoets.Common.Forms/Properties/Resources.Designer.cs	(.../Resources.Designer.cs)	(revision 3abef5ccb0ba5386c65d4156b319970fc4337a29)
+++ Ringtoets/Common/src/Ringtoets.Common.Forms/Properties/Resources.Designer.cs	(.../Resources.Designer.cs)	(revision 58c0d0a9d882210f47175f62ef1e4c494fce140b)
@@ -1,7 +1,7 @@
 //------------------------------------------------------------------------------
 // <auto-generated>
 //     This code was generated by a tool.
-//     Runtime Version:4.0.30319.18444
+//     Runtime Version:4.0.30319.17929
 //
 //     Changes to this file may cause incorrect behavior and will be lost if
 //     the code is regenerated.
@@ -239,6 +239,16 @@
         }
         
         /// <summary>
+        ///   Looks up a localized string similar to Bijdrage van de geselecteerde scenario&apos;s voor dit vak zijn opgeteld niet gelijk aan 100%.
+        /// </summary>
+        public static string FailureMechanismResultView_DataGridViewCellFormatting_Scenario_contribution_for_this_section_not_100 {
+            get {
+                return ResourceManager.GetString("FailureMechanismResultView_DataGridViewCellFormatting_Scenario_contribution_for_t" +
+                        "his_section_not_100", resourceCulture);
+            }
+        }
+        
+        /// <summary>
         ///   Looks up a localized string similar to Toetslaag 1.
         /// </summary>
         public static string FailureMechanismResultView_InitializeDataGridView_Assessment_layer_one {
Index: Ringtoets/Common/src/Ringtoets.Common.Forms/Properties/Resources.resx
===================================================================
diff -u -r3abef5ccb0ba5386c65d4156b319970fc4337a29 -r58c0d0a9d882210f47175f62ef1e4c494fce140b
--- Ringtoets/Common/src/Ringtoets.Common.Forms/Properties/Resources.resx	(.../Resources.resx)	(revision 3abef5ccb0ba5386c65d4156b319970fc4337a29)
+++ Ringtoets/Common/src/Ringtoets.Common.Forms/Properties/Resources.resx	(.../Resources.resx)	(revision 58c0d0a9d882210f47175f62ef1e4c494fce140b)
@@ -241,4 +241,7 @@
   <data name="FailureMechanismSectionResultIcon" type="System.Resources.ResXFileRef, System.Windows.Forms">
     <value>..\Resources\document-task.png;System.Drawing.Bitmap, System.Drawing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a</value>
   </data>
+  <data name="FailureMechanismResultView_DataGridViewCellFormatting_Scenario_contribution_for_this_section_not_100" xml:space="preserve">
+    <value>Bijdrage van de geselecteerde scenario's voor dit vak zijn opgeteld niet gelijk aan 100%</value>
+  </data>
 </root>
\ No newline at end of file
Index: Ringtoets/Common/src/Ringtoets.Common.Forms/Views/FailureMechanismResultView.cs
===================================================================
diff -u -rbe992ae906247ce3e6da6bd2bc31955610f1b528 -r58c0d0a9d882210f47175f62ef1e4c494fce140b
--- Ringtoets/Common/src/Ringtoets.Common.Forms/Views/FailureMechanismResultView.cs	(.../FailureMechanismResultView.cs)	(revision be992ae906247ce3e6da6bd2bc31955610f1b528)
+++ Ringtoets/Common/src/Ringtoets.Common.Forms/Views/FailureMechanismResultView.cs	(.../FailureMechanismResultView.cs)	(revision 58c0d0a9d882210f47175f62ef1e4c494fce140b)
@@ -123,6 +123,7 @@
             dataGridView.CurrentCellDirtyStateChanged += DataGridViewCurrentCellDirtyStateChanged;
             dataGridView.CellValidating += DataGridViewCellValidating;
             dataGridView.DataError += DataGridViewDataError;
+            dataGridView.CellFormatting += DataGridViewCellFormatting;
 
             var sectionName = new DataGridViewTextBoxColumn
             {
@@ -142,7 +143,8 @@
             {
                 DataPropertyName = "AssessmentLayerTwoA",
                 HeaderText = Resources.FailureMechanismResultView_InitializeDataGridView_Assessment_layer_two_a,
-                Name = "column_AssessmentLayerTwoA"
+                Name = "column_AssessmentLayerTwoA",
+                ReadOnly = true
             };
 
             assessmentLayerTwoB = new DataGridViewTextBoxColumn
@@ -197,7 +199,6 @@
 
         private void SetRowEditMode(DataGridViewRow row, bool checkboxSelected)
         {
-            row.Cells[assessmentLayerTwoA.Index].ReadOnly = checkboxSelected;
             row.Cells[assessmentLayerTwoB.Index].ReadOnly = checkboxSelected;
             row.Cells[assessmentLayerThree.Index].ReadOnly = checkboxSelected;
         }
@@ -262,10 +263,6 @@
                 {
                     return failureMechanismSectionResult.AssessmentLayerTwoA;
                 }
-                set
-                {
-                    failureMechanismSectionResult.AssessmentLayerTwoA = value;
-                }
             }
 
             public RoundedDouble AssessmentLayerTwoB
@@ -329,6 +326,18 @@
             }
         }
 
+        private void DataGridViewCellFormatting(object sender, DataGridViewCellFormattingEventArgs e)
+        {
+            if (e.ColumnIndex == assessmentLayerTwoA.Index && e.Value != null && double.IsNaN((RoundedDouble) e.Value))
+            {
+                dataGridView.Rows[e.RowIndex].Cells[e.ColumnIndex].ErrorText = Resources.FailureMechanismResultView_DataGridViewCellFormatting_Scenario_contribution_for_this_section_not_100;
+            }
+            else
+            {
+                dataGridView.Rows[e.RowIndex].Cells[e.ColumnIndex].ErrorText = string.Empty;
+            }
+        }
+
         #endregion
     }
 }
\ No newline at end of file
Index: Ringtoets/Common/test/Ringtoets.Common.Data.Test/FailureMechanism/FailureMechanismSectionResultTest.cs
===================================================================
diff -u -r3462d84f3304a35ba5ce26c7d3afc9c31cdf9205 -r58c0d0a9d882210f47175f62ef1e4c494fce140b
--- Ringtoets/Common/test/Ringtoets.Common.Data.Test/FailureMechanism/FailureMechanismSectionResultTest.cs	(.../FailureMechanismSectionResultTest.cs)	(revision 3462d84f3304a35ba5ce26c7d3afc9c31cdf9205)
+++ Ringtoets/Common/test/Ringtoets.Common.Data.Test/FailureMechanism/FailureMechanismSectionResultTest.cs	(.../FailureMechanismSectionResultTest.cs)	(revision 58c0d0a9d882210f47175f62ef1e4c494fce140b)
@@ -20,8 +20,10 @@
 // All rights reserved.
 
 using System;
+using Core.Common.Base.Data;
 using Core.Common.Base.Geometry;
 using NUnit.Framework;
+using Rhino.Mocks;
 using Ringtoets.Common.Data.FailureMechanism;
 
 namespace Ringtoets.Common.Data.Test.FailureMechanism
@@ -40,6 +42,10 @@
 
             // Assert
             Assert.AreSame(section, sectionResult.Section);
+            Assert.IsFalse(sectionResult.AssessmentLayerOne);
+            Assert.AreEqual((RoundedDouble)0, sectionResult.AssessmentLayerTwoA);
+            Assert.AreEqual((RoundedDouble)0, sectionResult.AssessmentLayerTwoB);
+            Assert.AreEqual((RoundedDouble)0, sectionResult.AssessmentLayerThree);
             CollectionAssert.IsEmpty(sectionResult.CalculationScenarios);
         }
 
@@ -54,6 +60,31 @@
             Assert.AreEqual("section", exception.ParamName);
         }
 
+        [Test]
+        public void AssessmentLayerTwoA_ScenariosDoNotAddUpToHunderdPercent_ThrowsArgumentException()
+        {
+            // Setup
+            var section = CreateSection();
+            var failureMechanismSectionResult = new FailureMechanismSectionResult(section);
+
+            var mocks = new MockRepository();
+            var calculationScenarioMock = mocks.StrictMock<ICalculationScenario>();
+            calculationScenarioMock.Expect(cs => cs.IsRelevant).Return(true);
+            calculationScenarioMock.Expect(cs => cs.Contribution).Return((RoundedDouble)50);
+
+            mocks.ReplayAll();
+
+            failureMechanismSectionResult.CalculationScenarios.Add(calculationScenarioMock);
+
+            // Call
+            var assessmentLayerTwoA = new RoundedDouble();
+            TestDelegate call = () => assessmentLayerTwoA = failureMechanismSectionResult.AssessmentLayerTwoA;
+
+            // Assert
+            Assert.Throws<ArgumentException>(call);
+            mocks.VerifyAll();
+        }
+
         private static FailureMechanismSection CreateSection()
         {
             var points = new[]
Index: Ringtoets/Common/test/Ringtoets.Common.Forms.Test/Views/FailureMechanismResultViewTest.cs
===================================================================
diff -u -re4a720dc28a93c2357d7e1b9702227cc645d46fb -r58c0d0a9d882210f47175f62ef1e4c494fce140b
--- Ringtoets/Common/test/Ringtoets.Common.Forms.Test/Views/FailureMechanismResultViewTest.cs	(.../FailureMechanismResultViewTest.cs)	(revision e4a720dc28a93c2357d7e1b9702227cc645d46fb)
+++ Ringtoets/Common/test/Ringtoets.Common.Forms.Test/Views/FailureMechanismResultViewTest.cs	(.../FailureMechanismResultViewTest.cs)	(revision 58c0d0a9d882210f47175f62ef1e4c494fce140b)
@@ -76,6 +76,7 @@
                 var dataGridView = (DataGridView) new ControlTester("dataGridView").TheObject;
 
                 Assert.AreEqual(5, dataGridView.ColumnCount);
+                Assert.IsTrue(dataGridView.Columns[assessmentLayerTwoAIndex].ReadOnly);
 
                 foreach (var column in dataGridView.Columns.OfType<DataGridViewComboBoxColumn>())
                 {
@@ -252,7 +253,6 @@
                     Assert.AreEqual(Color.FromKnownColor(KnownColor.ControlText), cellAssessmentLayerThreeForeColor);
                 }
 
-                Assert.AreEqual(checkBoxSelected, cellAssessmentLayerTwoA.ReadOnly);
                 Assert.AreEqual(checkBoxSelected, cellAssessmentLayerTwoB.ReadOnly);
                 Assert.AreEqual(checkBoxSelected, cellAssessmentLayerThree.ReadOnly);
             }
@@ -281,10 +281,6 @@
         }
 
         [Test]
-        [TestCase("1", assessmentLayerTwoAIndex, "AssessmentLayerTwoA")]
-        [TestCase("1e-6", assessmentLayerTwoAIndex, "AssessmentLayerTwoA")]
-        [TestCase("1e+6", assessmentLayerTwoAIndex, "AssessmentLayerTwoA")]
-        [TestCase("14.3", assessmentLayerTwoAIndex, "AssessmentLayerTwoA")]
         [TestCase("1", assessmentLayerTwoBIndex, "AssessmentLayerTwoB")]
         [TestCase("1e-6", assessmentLayerTwoBIndex, "AssessmentLayerTwoB")]
         [TestCase("1e+6", assessmentLayerTwoBIndex, "AssessmentLayerTwoB")]
@@ -342,6 +338,68 @@
             }
         }
 
+        [Test]
+        public void FailureMechanismResultView_AssessmentLayerTwoANaN_ShowsErrorTooltip()
+        {
+            // Setup
+            var mocks = new MockRepository();
+            var calculationScenarioMock = mocks.StrictMock<ICalculationScenario>();
+            calculationScenarioMock.Expect(cs => cs.Contribution).Return((RoundedDouble) 0.3);
+            calculationScenarioMock.Expect(cs => cs.IsRelevant).Return(true);
+
+            mocks.ReplayAll();
+
+            var rowIndex = 0;
+
+            using (var view = ShowFullyConfiguredFailureMechanismResultsView())
+            {
+                var sections = (List<FailureMechanismSectionResult>)view.Data;
+                sections[0].CalculationScenarios.Add(calculationScenarioMock);
+
+                var gridTester = new ControlTester("dataGridView");
+                var dataGridView = (DataGridView)gridTester.TheObject;
+
+                DataGridViewCell dataGridViewCell = dataGridView.Rows[rowIndex].Cells[assessmentLayerTwoAIndex];
+
+                // Call
+                var formattedValue = dataGridViewCell.FormattedValue; // Need to do this to fire the CellFormatting event.
+
+                // Assert
+                Assert.AreEqual("Bijdrage van de geselecteerde scenario's voor dit vak zijn opgeteld niet gelijk aan 100%", dataGridViewCell.ErrorText);
+            }
+        }
+
+        [Test]
+        public void FailureMechanismResultView_AssessmentLayerTwoAHasValue_DoesNotShowsErrorTooltip()
+        {
+            // Setup
+            var mocks = new MockRepository();
+            var calculationScenarioMock = mocks.StrictMock<ICalculationScenario>();
+            calculationScenarioMock.Expect(cs => cs.Contribution).Return((RoundedDouble)1.0);
+            calculationScenarioMock.Expect(cs => cs.IsRelevant).Return(true);
+
+            mocks.ReplayAll();
+
+            var rowIndex = 0;
+
+            using (var view = ShowFullyConfiguredFailureMechanismResultsView())
+            {
+                var sections = (List<FailureMechanismSectionResult>)view.Data;
+                sections[0].CalculationScenarios.Add(calculationScenarioMock);
+
+                var gridTester = new ControlTester("dataGridView");
+                var dataGridView = (DataGridView)gridTester.TheObject;
+
+                DataGridViewCell dataGridViewCell = dataGridView.Rows[rowIndex].Cells[assessmentLayerTwoAIndex];
+
+                // Call
+                var formattedValue = dataGridViewCell.FormattedValue; // Need to do this to fire the CellFormatting event.
+
+                // Assert
+                Assert.AreEqual(string.Empty, dataGridViewCell.ErrorText);
+            }
+        }
+
         private const int nameColumnIndex = 0;
         private const int assessmentLayerOneIndex = 1;
         private const int assessmentLayerTwoAIndex = 2;