Skip to the conclusion/discussion, that's where you'll understand this study.
The pharmacokinetic measurements and statistics are irrelevant to the reader, it's just to add credibility and specifics to the study because it's a broad study and the researchers need to leave little to no room for error.

First off let's note, on average of only 19 people, they smoked upward of 10 joints/day for the past 10 years.. that's VERY chronic & heavy smoking lol.
They also even note they didn't control for variables such as education, economic status, lifestyle/health, etc between the cannabis using group and the control.. so another fact to take this study with a grain of salt.

Essentially, they noted after stopping cannabis use for 3 weeks, the cannabis group improved in performance in critical tasking and general attentiveness, yet they were still below the control group. Don't place merit in this. The use was extremely above the norm.. and only 3 weeks afer that kind of use isn't really enough. I can tell you with certainty they will go back to baseline soonafter though. Furthermore, this corroborates what I say about one of the few deleterious components of chronic/heavy use, aka decreased attentiveness.