To highlight something from ASI that I quoted in my last post, having a reliable standard is very important. It's not enough to say that one just bought a fresh kit or is using one that everyone says is accurate, before anyone gives much weight to one side or the other, results should be compared to a known standard. I don't think making a standard is as simple as ASI made it sound and depending on the kit, testing SSW is not always straight forward (http://www.athiel.com/lib6/cal.htm), but unless one is comparing like method of analysis and has a reliable standard, they can never be sure who is right. For someone who is really serious about this, Hach sells a 1000 ppm Ca standard. Its a lot easier to precisely dilute a standard than it would be to make a good one from scratch.