SSRS Median and drill down reports - sql-server

Question

I have a SQL Server Reporting Services report in which I calculate the median using an ArrayList in a custom code block as median is not a built in function in SSRS. The median and average are displayed in a column chart. The report includes a drilldown to a detail report. If I drill down to the detail report and then hit the back arrow in the browser to navigate back to the master report, the median on the master report is not being calculated. That column of the column chart is empty while the average is displayed as it should.
Does anyone have any thoughts on why this is happening or how I can fix it?
The code is below. AddValue() first creates a new SortedList of it doesn't already exist. It then adds the newValue to an ArrayList keyed by the string passed to the function, creating one if it didn't previously exist. Each ArrayList represents a different set of values for which we would want to calculate the median.
GetMedian() calculates the median for the ArrayList identified by the passed string. GetMedian() is used in the value for the column in the chart.
Public Dim ClearanceList As System.Collections.SortedList
Function AddValue(ByVal whichList As String, ByVal newValue As Integer) As Decimal
Dim thisList As System.Collections.ArrayList
If (ClearanceList is Nothing) Then
ClearanceList = New System.Collections.SortedList
End If
If (ClearanceList.ContainsKey(whichList)) Then
'Get a reference to the desired ArrayList and add the new value to it.
thisList = ClearanceList.GetByIndex(ClearanceList.IndexOfKey(whichList))
Else
thisList = New System.Collections.ArrayList()
' Create a new element in the SortedList
ClearanceList.Add(whichList, thisList)
End If
thisList.Add(newValue)
AddValue = thisList.Count
End Function
Function GetMedian(ByVal whichList As String) As Decimal
Dim thisList As System.Collections.ArrayList
Dim count As Integer
thisList = ClearanceList.GetByIndex(ClearanceList.IndexOfKey(whichList))
count = thisList.count
If (count > 0) Then
thisList.Sort()
If count Mod 2 = 1 Then
GetMedian = thisList((count - 1) / 2)
Else
GetMedian = (thisList((count / 2) - 1) + thisList((count / 2))) / 2
End If
Else
GetMedian = -1
End If
End Function