Sets or returns the type of calculation used to summarize a data field. The available types are determined by SummarizeByType.

 public SummarizeByType SummarizeBy{ get; set; }

Public Property SummarizeBy() As SummarizeByType


          ExcelApplication xla = new ExcelApplication();
          Workbook wb = xla.Open(@"C:\MySpreadsheet.xls");
          Worksheet ws = wb.Worksheets[0];
          DataField df = ws.PivotTables[0].DataFields[0];
          df.SummarizeBy = PivotTableField.SummarizeByType.Average;
        

          Dim xla As New ExcelApplication()
          Dim wb As Workbook = xla.Open("C:\MySpreadsheet.xls")
          Dim ws As Worksheet = wb.Worksheets(0)
          Dim df As DataField = ws.PivotTables(0).DataFields(0)
          df.SummarizeBy = PivotTableField.SummarizebyType.Average;

DataFields are set to Count for string values and Sum for numerical values by default.