So if I understand the chart, because the i1's which are the same as the units furnished with the NEC Spectraview II are closest to the Jeti reference, they're the most accurate?
The Jeti is the reference device. It’s
very expensive, it’s
very accurate. The owner very, very likely had it recalibrated and certified yearly. Those devices that correlate to it closely are ideal because they correlate with a high end reference. Those that don’t are not accurate devices.
You buy a 25 foot tape measure at
Home Depot and you assume, expect it is accurate. How accurate is that, how accurate do you need? You compare it to some $10K super duper NASA reference measuring system and see it’s accurate to within +/- 1000th of an inch, but another tape measure you bought at
Harbor Freight is accurate within a 10th of an inch. You now know which of the two tape measures is more accurate. And of course it’s useful to determine how accurate you need a tape measure (maybe 10th of an inch accuracy if more than enough for the task at hand). But if the two tape measures are about the same price, you’d be foolish to not recommend or buy the tape measure from
Home Depot. And this is what the graph is illustrating.