function checkContribution(formulier) {
  var msg = "";
  if (!hasInput(formulier.firstname)) {
      msg += "Please fill in your firstname.\n";
  }

  if (!hasInput(formulier.lastname)) {
      msg += "Please fill in your lastname.\n";
  }

  if (hasInput(formulier.email)) {
    if (!emailCheck(formulier.email.value)) {
      msg += "This is not a valid email-adres. Please provide a valid one\n";
    }
  } else {
      msg += "Please fill in your e-mail address.\n";
  }

  if (!hasInput(formulier.title)) {
      msg += "Please provide a title for your contribution.\n";
  }

  if (!hasInput(formulier.theme)) {
      msg += "Please choose a theme for your contribution.\n";
  }
  
  if (!hasInput(formulier.type)) {
      msg += "Please choose a type of contribution.\n";
  }
  
  if (!hasInput(formulier.summary)) {
      msg += "Please write a short summary.\n";
  }

  if (!hasInput(formulier.abstract)) {
      msg += "Please write an abstract.\n";
  }

  if (!hasInput(formulier.bio)) {
      msg += "Please write a short bio.\n";
  }
  
  if(msg) {
      alert(msg);
      return false;
  }
}