function threshold() {
var a = document.calc.npvcost.value
var c = document.calc.appcost.value
var e = document.calc.headnumber.value
var f = document.calc.cropvalue.value
var d = document.calc.rowspace.value

if (isNumber(a)==false) {
alert('Please enter a valid number.');
return false;
}

if (isNumber(c)==false) {
alert('Please enter a valid number.');
return false;
}

if (isNumber(d)==false) {
alert('Please enter a valid number.');
return false;
}

if (isNumber(e)==false) {
alert('Please enter a valid number.');
return false;
}

if (isNumber(f)==false) {
alert('Please enter a valid number.');
return false;
}

a=parseFloat(a);
c=parseFloat(c);
d=parseFloat(d);
e=parseFloat(e);
f=parseFloat(f);

if (f<=0) {
alert('Crop value is invalid.');
return false;
}

if (e<=0) {
alert('Head number is invalid.');
return false;
}

var result = ((a+c)*d)/(e*f*2.4);
var result = result.toFixed(1);
if (isNaN(result)) {
alert('Please make sure there are no values missing.');
return false;
}
if (result<=1) {suffix=" larva per head"}
else {suffix=" larvae per head"};
document.calc.elements[5].value = result + suffix;

return false;

function isNumber(sText)
{
   var ValidChars = "0123456789.";
   var IsNumber=true;
   var Char;
   for (i = 0; i < sText.length && IsNumber == true; i++) 
      { 
      Char = sText.charAt(i); 
      if (ValidChars.indexOf(Char) == -1) 
         {
         IsNumber = false;
         }
      }
   return IsNumber; 
   }

}