How to get a selected index of a dropdownlist by using jquery in MVC3?

$(document).ready(function () {
        $("#ddlPA").change(function () {

            var url = "Reports/Models";
            var ddlsource = "#ddlPA";
            var ddltarget = "#ddlPSA";
            var selectedPSA = $(this).val();
            
            $.getJSON('@Url.Action("Models")', { param: $(ddlsource).val(), id: selectedPSA },
                 function (data) {
                     $(ddltarget).empty();
                     $.each(data, function (index, optionData) {
                         $(ddltarget).append("<option value='" + optionData.Text + "'>" + optionData.Value + "</option>");
                     });

                 });

                 if (selectedPSA = null || selectedPSA == '') {
                     $(ddltarget).hide();
                 }
                 else
                 {
                     $(ddltarget).show();

                 }

        });

    });