Advanced Data Grid with Count only returns zero

i'm getting started flex , having problems getting advanceddatagrid count total number of records group. have following control in studentcount column 0 grouped rows. can point me in right direction?

thanks!

 

<mx:panel x="75" y="10" width="933" height="797" layout="absolute">       
       
        <mx:advanceddatagrid x="65" y="10" id="adg1" designviewdatatype="tree" width="821.5"  enabled="true" initialize="gc.refresh();" height="406" selectionmode="singlecell" editable="true">
        <mx:dataprovider>
            <mx:groupingcollection id="gc" source="{dataxml.row}">
           
                    <mx:grouping>
                        <mx:groupingfield name="appls_area_of_interest_desc"/>
                   
                        <mx:groupingfield name="appls_apdc_desc">
                               <mx:summaryrow summaryplacement="group">
                                      <mx:summaryfield datafield="appls_ssn_frmt"
                                        label="studentcount" operation="count"/>
                                  </mx:summaryrow>
                        </mx:groupingfield>
                    </mx:grouping>
           
            </mx:groupingcollection>
        </mx:dataprovider>
            <mx:columns>
                <mx:advanceddatagridcolumn headertext="interest desc" datafield="appls_area_of_interest_desc"/>
                <mx:advanceddatagridcolumn headertext="interest code" datafield="appls_area_of_interest_code"/>
                <mx:advanceddatagridcolumn headertext="total students" datafield="studentcount"/>
                <mx:advanceddatagridcolumn headertext="darton id" datafield="appls_ssn_frmt"/>
                <mx:advanceddatagridcolumn headertext="name" datafield="appls_fullname_lfm"/>
                <mx:advanceddatagridcolumn headertext="phone" datafield="appls_phone_number"/>
                <mx:advanceddatagridcolumn headertext="apdc desc" datafield="appls_apdc_desc"/>
                <mx:advanceddatagridcolumn headertext="apdc code" datafield="appls_apdc_code"/>
            </mx:columns>
        </mx:advanceddatagrid>
    </mx:panel>
</mx:application>

i guess count function works on numeric feilds.

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