Checkbox values problem in gridview

i can explain for my doubt with example.see.my table have multiple types of employee details like as management,Programmer,Designers,Accountants,Contra ctEmployees.Each type of department having many employee details.Now my drop down list having types of department names like that above types of department names..if i select any one type of department using dropdown list then employee details are display in gridview format .gridview data column as employee Name,Address,Phone_No,email_address and each every row with checkboxes.gridview showing data according to select the dept.No problem.

first I select Mangemment type of employee grid view showing that type of employee details ,here i checked first two employee details then checkbox values put in one viewstate,then i go to Programmer type of employee it showing that type employee details here i want to checked two employee details using checkbox but what happened first two checkboxes are already checked. so but view state having proper checked check box values but problem is
whenever i go to select for another type of employee details checkboxes are checked based upon order of previous checked index.




i checked checkbox values of dropdown list selected index changed event,values are coming from viewstate,so we want viewstate values add plus new checked checkbox values ,but whenever i select another type of employee using drop down list view state values coming then adding but viewstate value one is removing then unchecked checkbox value one is adding.so data ll b getting for unproperly.

i cant find the which type of error in this.checkbox values or drop down list or gridview .

Please help me

thanigaimani.thirumalai