[slg]

That is definitely true. If you want to measure ref bias, you should only consider plays in which the refs are in control of the spot (basically plays that end in a tackle being awarded).

There is also the flaw in that these measurements are reported by humans so we can't tell the source of any bias that might exist in the numbers. Similar to the glasses of water question in the lede, I think it is more likely that any bias comes on the reporting end rather than in the actual data. The refs don't spot the ball always on yard line so they might spot the ball at 38.4 yards. The person reporting that value is judging it by eye, so they will generally report that as on the 38 but will occasionally report it as being on the 39. Does the chance they report a ball that is at 39.4 yards as being on the 40 yard line increase in comparison?