1) ASS isn't a PFF stat, and the stats mentioned in this tweet aren't at all analogous to PFF player grades, nor have a charting process at all related to anything being said in this video 2) A first read refers to a side/area of the field that the QB looks at first. There's obvious uncertainty here, but we have always acknowledged this from the first article we ever published on the topic. Despite that, we've always found the version of that stat that we chart to be slightly more predictive of production than regular target shares. (Though the two stats are usually very similar for most WRs!) 3) ASS only tries to measure one aspect of WR play. It is less predictive of production on its own than basically every good volume/production stat for this reason. (We've never been shy about saying this, either). It's very good at predicting whether a receiver will be targeted on a route, but it can't tell you anything about the volume/variety/difficulty of routes that receiver can run, their catch radius/ball skills, anything they can do after the catch, or anything else besides whether they gained separation on that play. Links to detailed reading on all of this are below, for anyone who's interested.