A Simple Measurement
Before you try to measure something as complex as people's performance, make sure you can make simple measurements reliably. Length, for example. The length of the coast of Maine.
How long is the coast of Maine? If you draw a straight line from one end of the coast to the other and measure the length of the line, it's about 220 miles. That's one measurement.
Now measure a different way. Take a string that's ten miles long. Hold one end of it at the southern end of the coast. Stretch the string straight and find where the other end of the string touches the coast. Continue measuring these ten-mile lengths and add them up. The length of the coast is maybe 400 miles.
Next, use a one-mile string, then 1000 feet, then 100 feet. The shorter the string, the longer the coast.
If you use a very short string, say one foot, it is difficult to decide even where the coast is, or whether "coast" means anything at all. Where does the land end and the ocean begin? How do you measure around the mouth of the Penobscot River? What about all of those islands? Do you measure around those or not? And when you measure at such a small scale, the coast is moving as you measure it. Should you measure at high tide or at low tide?
As you measure at still smaller scales, say the width of an oxygen molecule, "coast" becomes even less meaningful. At a small enough scale, the "coast" is discontinuous. At that point, even if "coast" meant something, the notion of "length" may no longer apply.
Many sources claim that the coast of Maine is more than 3000 miles long. What does that mean? The number is nearly meaningless if you don't know how it was measured.
The length of the coast of Maine depends almost entirely on how you measure it. Why would measuring people be any different?