Index: Ringtoets/MacroStabilityInwards/src/Ringtoets.MacroStabilityInwards.Data/SoilProfile/MacroStabilityInwardsStochasticSoilProfile.cs
===================================================================
diff -u -r9f936b0b29f490a024df02345df182133b1c041a -r6545e28fb366f9bfdf71e776a404e9ce256df182
--- Ringtoets/MacroStabilityInwards/src/Ringtoets.MacroStabilityInwards.Data/SoilProfile/MacroStabilityInwardsStochasticSoilProfile.cs	(.../MacroStabilityInwardsStochasticSoilProfile.cs)	(revision 9f936b0b29f490a024df02345df182133b1c041a)
+++ Ringtoets/MacroStabilityInwards/src/Ringtoets.MacroStabilityInwards.Data/SoilProfile/MacroStabilityInwardsStochasticSoilProfile.cs	(.../MacroStabilityInwardsStochasticSoilProfile.cs)	(revision 6545e28fb366f9bfdf71e776a404e9ce256df182)
@@ -100,6 +100,19 @@
             Probability = fromProfile.Probability;
         }
 
+        /// <summary>
+        /// Updates the probability of the <see cref="MacroStabilityInwardsStochasticSoilProfile"/> 
+        /// by adding <paramref name="probabilityToAdd"/>.
+        /// </summary>
+        /// <param name="probabilityToAdd">The amount to increase the <see cref="Probability"/>
+        /// with.</param>
+        /// <exception cref="ArgumentOutOfRangeException">Thrown when the <paramref name="probabilityToAdd"/>
+        /// is outside the range [0, 1].</exception>
+        public void AddProbability(double probabilityToAdd)
+        {
+            Probability += probabilityToAdd;
+        }
+
         public override string ToString()
         {
             return SoilProfile?.ToString() ?? string.Empty;
Index: Ringtoets/MacroStabilityInwards/test/Ringtoets.MacroStabilityInwards.Data.Test/SoilProfile/MacroStabilityInwardsStochasticSoilProfileTest.cs
===================================================================
diff -u -r9f936b0b29f490a024df02345df182133b1c041a -r6545e28fb366f9bfdf71e776a404e9ce256df182
--- Ringtoets/MacroStabilityInwards/test/Ringtoets.MacroStabilityInwards.Data.Test/SoilProfile/MacroStabilityInwardsStochasticSoilProfileTest.cs	(.../MacroStabilityInwardsStochasticSoilProfileTest.cs)	(revision 9f936b0b29f490a024df02345df182133b1c041a)
+++ Ringtoets/MacroStabilityInwards/test/Ringtoets.MacroStabilityInwards.Data.Test/SoilProfile/MacroStabilityInwardsStochasticSoilProfileTest.cs	(.../MacroStabilityInwardsStochasticSoilProfileTest.cs)	(revision 6545e28fb366f9bfdf71e776a404e9ce256df182)
@@ -87,6 +87,58 @@
         }
 
         [Test]
+        [SetCulture("nl-NL")]
+        [TestCase(-1)]
+        [TestCase(1)]
+        [TestCase(-0.51)]
+        [TestCase(0.51)]
+        [TestCase(double.NaN)]
+        public void AddProbability_ProbabilityToAddResultsInvalidProbability_ThrowsArgumentOutOfRangeException(double probabilityToAdd)
+        {
+            // Setup
+            const double probability = 0.5;
+
+            var mocks = new MockRepository();
+            var soilProfile = mocks.Stub<IMacroStabilityInwardsSoilProfile>();
+            mocks.ReplayAll();
+
+            var profile = new MacroStabilityInwardsStochasticSoilProfile(probability, soilProfile);
+
+            // Call
+            TestDelegate test = () => profile.AddProbability(probabilityToAdd);
+
+            // Assert
+            const string expectedMessage = "Het aandeel van de ondergrondschematisatie in het stochastische ondergrondmodel" +
+                                           " moet in het bereik [0,0, 1,0] liggen.";
+            TestHelper.AssertThrowsArgumentExceptionAndTestMessage<ArgumentOutOfRangeException>(test, expectedMessage);
+            mocks.VerifyAll();
+        }
+
+        [Test]
+        [TestCase(0.5)]
+        [TestCase(-0.5)]
+        [TestCase(-0.2)]
+        [TestCase(0.2)]
+        public void AddProbability_ProbabilityToAddResultsValidProbability_ReturnsExpectedProbability(double probabilityToAdd)
+        {
+            // Setup
+            const double probability = 0.5;
+
+            var mocks = new MockRepository();
+            var soilProfile = mocks.Stub<IMacroStabilityInwardsSoilProfile>();
+            mocks.ReplayAll();
+
+            var profile = new MacroStabilityInwardsStochasticSoilProfile(probability, soilProfile);
+
+            // Call
+            profile.AddProbability(probabilityToAdd);
+
+            // Assert
+            Assert.AreEqual(probability + probabilityToAdd, profile.Probability, 1e-6);
+            mocks.VerifyAll();
+        }
+
+        [Test]
         public void Update_WithNullProfile_ThrowsArgumentNullException()
         {
             // Setup
Index: Ringtoets/Piping/src/Ringtoets.Piping.Data/SoilProfile/PipingStochasticSoilProfile.cs
===================================================================
diff -u -ref905ca4d14236014d884b5658e18838b697eb01 -r6545e28fb366f9bfdf71e776a404e9ce256df182
--- Ringtoets/Piping/src/Ringtoets.Piping.Data/SoilProfile/PipingStochasticSoilProfile.cs	(.../PipingStochasticSoilProfile.cs)	(revision ef905ca4d14236014d884b5658e18838b697eb01)
+++ Ringtoets/Piping/src/Ringtoets.Piping.Data/SoilProfile/PipingStochasticSoilProfile.cs	(.../PipingStochasticSoilProfile.cs)	(revision 6545e28fb366f9bfdf71e776a404e9ce256df182)
@@ -98,6 +98,19 @@
             Probability = fromProfile.Probability;
         }
 
+        /// <summary>
+        /// Updates the probability of the <see cref="PipingStochasticSoilProfile"/> 
+        /// by adding <paramref name="probabilityToAdd"/>.
+        /// </summary>
+        /// <param name="probabilityToAdd">The amount to increase the <see cref="Probability"/>
+        /// with.</param>
+        /// <exception cref="ArgumentOutOfRangeException">Thrown when the <paramref name="probabilityToAdd"/>
+        /// is outside the range [0, 1].</exception>
+        public void AddProbability(double probabilityToAdd)
+        {
+            Probability += probabilityToAdd;
+        }
+
         public override string ToString()
         {
             return SoilProfile.ToString();
Index: Ringtoets/Piping/test/Ringtoets.Piping.Data.Test/SoilProfile/PipingStochasticSoilProfileTest.cs
===================================================================
diff -u -r6cf604a94300560817de70fd42d09baa5b22c3a6 -r6545e28fb366f9bfdf71e776a404e9ce256df182
--- Ringtoets/Piping/test/Ringtoets.Piping.Data.Test/SoilProfile/PipingStochasticSoilProfileTest.cs	(.../PipingStochasticSoilProfileTest.cs)	(revision 6cf604a94300560817de70fd42d09baa5b22c3a6)
+++ Ringtoets/Piping/test/Ringtoets.Piping.Data.Test/SoilProfile/PipingStochasticSoilProfileTest.cs	(.../PipingStochasticSoilProfileTest.cs)	(revision 6545e28fb366f9bfdf71e776a404e9ce256df182)
@@ -84,6 +84,50 @@
         }
 
         [Test]
+        [SetCulture("nl-NL")]
+        [TestCase(-1)]
+        [TestCase(1)]
+        [TestCase(-0.51)]
+        [TestCase(0.51)]
+        [TestCase(double.NaN)]
+        public void AddProbability_ProbabilityToAddResultsInvalidProbability_ThrowsArgumentOutOfRangeException(double probabilityToAdd)
+        {
+            // Setup
+            const double probability = 0.5;
+
+            PipingSoilProfile profile = PipingSoilProfileTestFactory.CreatePipingSoilProfile();
+            var stochasticSoilProfile = new PipingStochasticSoilProfile(probability, profile);
+
+            // Call
+            TestDelegate test = () => stochasticSoilProfile.AddProbability(probabilityToAdd);
+
+            // Assert
+            const string expectedMessage = "Het aandeel van de ondergrondschematisatie in het stochastische ondergrondmodel" +
+                                           " moet in het bereik [0,0, 1,0] liggen.";
+            TestHelper.AssertThrowsArgumentExceptionAndTestMessage<ArgumentOutOfRangeException>(test, expectedMessage);
+        }
+
+        [Test]
+        [TestCase(0.5)]
+        [TestCase(-0.5)]
+        [TestCase(-0.2)]
+        [TestCase(0.2)]
+        public void AddProbability_ProbabilityToAddResultsValidProbability_ReturnsExpectedProbability(double probabilityToAdd)
+        {
+            // Setup
+            const double probability = 0.5;
+
+            PipingSoilProfile profile = PipingSoilProfileTestFactory.CreatePipingSoilProfile();
+            var stochasticSoilProfile = new PipingStochasticSoilProfile(probability, profile);
+
+            // Call
+            stochasticSoilProfile.AddProbability(probabilityToAdd);
+
+            // Assert
+            Assert.AreEqual(probability + probabilityToAdd, stochasticSoilProfile.Probability, 1e-6);
+        }
+
+        [Test]
         public void Update_SoilProfileNull_ThrowsArgumentNullException()
         {
             // Setup