function radio_selected(radiobutton) {
 for (i = 0; i < radiobutton.length; i++) {
  if (radiobutton[i].checked) {
   return true;
  }
 }
 return false;
}
function valid_email(email) {
 var filter  = /^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/;
 if (filter.test(email)) {
  return true;
 } else {
  return false;
 }
}
function check_form(page) {
if ( (page == 1) && (!document.prontosurvey.q1.value) ) {
 alert('Please write your name');
 return false;
}
if ( (page == 1) && (!document.prontosurvey.q2.value) ) {
 alert('Please write your last name');
 return false;
}
if ( (page == 1) && (!valid_email(document.prontosurvey.q3.value)) ) {
 alert('Write a valid e-mail please.');
 return false;
}
if ( (page == 1) && (!radio_selected(document.prontosurvey.q4)) ) {
 alert('Choose your gender please');
 return false;
}
if ( (page == 1) && (!radio_selected(document.prontosurvey.q5)) ) {
 alert('Choose your age please');
 return false;
}
if ( (page == 1) && (!radio_selected(document.prontosurvey.q6)) ) {
 alert('* Value required');
 return false;
}
if ( (page == 1) && (!radio_selected(document.prontosurvey.q7)) ) {
 alert('* Value Required');
 return false;
}
if ( (page == 1) && (!radio_selected(document.prontosurvey.q8)) ) {
 alert('* Value Required');
 return false;
}
if ( (page == 1) && (!radio_selected(document.prontosurvey.q9)) ) {
 alert('* Value Required');
 return false;
}
if ( (page == 1) && (!radio_selected(document.prontosurvey.q10)) ) {
 alert('* Value Required');
 return false;
}
if ( (page == 1) && (!radio_selected(document.prontosurvey.q11)) ) {
 alert('* Value Required');
 return false;
}
if ( (page == 1) && (!radio_selected(document.prontosurvey.q12)) ) {
 alert('* Value Required');
 return false;
}
if ( (page == 1) && (!radio_selected(document.prontosurvey.q13)) ) {
 alert('* Value Required');
 return false;
}
if ( (page == 1) && (!radio_selected(document.prontosurvey.q14)) ) {
 alert('* Value Required');
 return false;
}
if ( (page == 1) && (!radio_selected(document.prontosurvey.q15)) ) {
 alert('* Value Required');
 return false;
}
if ( (page == 1) && (!radio_selected(document.prontosurvey.q16)) ) {
 alert('* Value Required');
 return false;
}
if ( (page == 1) && (!radio_selected(document.prontosurvey.q17)) ) {
 alert('* Value Required');
 return false;
}
if ( (page == 1) && (!radio_selected(document.prontosurvey.q18)) ) {
 alert('* Value Required');
 return false;
}
if ( (page == 1) && (!radio_selected(document.prontosurvey.q19)) ) {
 alert('* Value Required');
 return false;
}

}