Please can someone tell me how to add an item to a CheckBoxList and then select it?

Code:
                <div class="seasonType">
                    Analysis Type<br />
                    <asp:CheckBoxList ID="cblAnalType" runat="server">
                    </asp:CheckBoxList>
                </div>

Code:
            DS = recall.getDistinctType()

            dt = DS.Tables(0)

            For Each dr In dt.Rows
                cblAnalType.Items.Add(dr("RG_TYPE_VALUE").ToString)
                If CBool(dr("RG_TYPE_SET")) Then
                    cblAnalType.SelectedValue = dr("RG_TYPE_VALUE").ToString
                End If
            Next
I have written this code but the RG_TYPE_SET is a boolean, so if it is true I want the tick boc to be selected.

When I run this it only ever selects the last item in the list! Can more than one item be selected?

thanks Paul