C:/Documents and Settings/Stephen/Desktop/Biostatistics Library and Program/Biostatistics/T-TestGroupsCollection.cs

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using System;
using System.Collections;
using System.Collections.Generic;

namespace StephenAshley.Biostatistics
{
   using NUMBER = Decimal;

   public class T_TestGroupsCollection : GroupsCollection, IEnumerable
   {
      #region *** constructors ***
      public T_TestGroupsCollection()
         : base()
      {
      }

      public T_TestGroupsCollection(List<Group> lstGroups)
         : base(lstGroups)
      {
         if (k() > 2)
         {
            throw new BiostatisticsException(
               "T_TestGroupsCollection contains more than 2 Groups.");
         }
      }
      #endregion
      #region ***** methods ******
      public override void Add(Group group)
      {
         if (group == null)
         {
            throw new BiostatisticsException(
               "group parameter in Add method of T_TestGroupsCollection is null.");
         }

         if (groupsArray == null || (k() == 1))
            base.Add(group);
         else
            throw new BiostatisticsException(
   "Addition of Group would result in T_TestGroupsCollection with more than 2 Groups.");
      }

      public override void AddRange(List<Group> lstGroups)
      {
         if (lstGroups == null)
         {
            throw new BiostatisticsException(
               "groups parameter in AddRange method of T_TestGroupsCollection is null.");
         }

         if (groupsArray == null || (k() + lstGroups.Count <= 2))
            base.AddRange(lstGroups);
         else
            throw new BiostatisticsException(
"Addition of a Group would result in a T_TestGroupsCollection with more than 2 Groups.");
      }

      public NUMBER EstimatedPopulationVariance()
      {
         if (k() != 2)
            throw new BiostatisticsException(
         "T-TestGroupsList does not contain 2 Groups.");
         Group groupA = groupsArray[0];
         Group groupB = groupsArray[1];
         NUMBER decEstimatedPopulationVariance;
         try
         {
            decEstimatedPopulationVariance = (groupA.SumOfSquaredDeviates() 
               + groupB.SumOfSquaredDeviates()) / (groupA.n() + groupB.n() - 2);
         }
         catch (Exception e)
         {
            throw new BiostatisticsException(e.Source + ": " + e.Message, e);
         }
         return decEstimatedPopulationVariance;
      }

      public NUMBER EstimatedStandardErrorOfDifferenceOfMeans()
      {
         if (k() != 2)
            throw new BiostatisticsException(
         "T-TestGroupsList does not contain 2 Groups.");
         NUMBER decEstimatedStandardErrorOfDifferenceOfMeans;
         try
         {
            decEstimatedStandardErrorOfDifferenceOfMeans = 
               DecMath.Sqrt(EstimatedPopulationVariance() / groupsArray[0].n() +
               EstimatedPopulationVariance() / groupsArray[1].n());
         }
         catch (Exception e)
         {
            throw new BiostatisticsException(e.Source + ": " + e.Message, e);
         }
         return decEstimatedStandardErrorOfDifferenceOfMeans;
      }

      public virtual NUMBER t()
      {
         if (k() != 2)
            throw new BiostatisticsException(
         "T-TestGroupsList does not contain 2 Groups.");
         NUMBER decT;
         try
         {
            decT = (groupsArray[0].Mean() - groupsArray[1].Mean()) /
            EstimatedStandardErrorOfDifferenceOfMeans();
         }
         catch (Exception e)
         {
            throw new BiostatisticsException(e.Source + ": " + e.Message, e);
         }
         return decT;
      }

      public Int32 DegreesOfFreedom()
      {
         if (k() != 2)
            throw new BiostatisticsException(
         "T-TestGroupsList does not contain 2 Groups.");
         return groupsArray[0].n() + groupsArray[1].n() - 2;
      }

      public virtual NUMBER p()
      { return Distributions.ProbabilityT(t(), DegreesOfFreedom()); }

      #endregion

   }
}

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