Add return false; after your ajax request to prevent the page from refreshing.
Code Snippet
$(document).ready(function() {
  $("#submitbtn").click(function() {
    var first = $("#name").val();
    var last = $("#last").val();
    var sin = $("#sin").val();
    var pwd = $("pwd").val();
    $.ajax({
      type: "POST",
      data: {first:first,last:last,sin:sin,pwd:pwd},
      url: "addemployee.php",
      success: function(result) {
        $("#resultadd").html(response);
      }
});
return false; }); });
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solved how to keep the page from refreshing? jQuery Ajax PHP