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

 public SummarizeByType SummarizeBy{ get; set; }

Public Property SummarizeBy() As SummarizeByType


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

Data fields are set to 'count' for string values and 'sum' for numerical values by default.

This property cannot be set on values labels and such fields will return SummarizeByType.Sum by default. You can check if a PivotTable field is values label with the IsValuesLabel property