From Behindthenet.ca: Junior Hockey Projections
Twenty years ago, almost all every NHL player was drafted from the Canadian Hockey League. Even today, the majority of NHL draft picks still come from the three Tier I junior hockey leagues, the Western Hockey League (WHL), the Ontario Hockey League (OHL) and the Quebec Major Junior Hockey League (QMJHL). But how does player performance translate from these leagues to the NHL?
It is obvious that, despite drafting thousands of times, NHL scouting hasn’t adequately answered this question. Looking at the 14th-23rd post-expansion drafts, from 1979 to 1989, 15 of 33 picks had careers that lasted more than 1000 games. But NHL teams also used the second and third picks on players like Dave Chyzowski and Neil Brady, not to mention famous "busts" Doug Wickenheiser, Doug Smith and Perry Turnbull. Even after 20+ years of evaluating CHL players, some teams looked at Vincent Damphousse and Adam Foote and decided they’d be better off with Brady and Chyzowski.
So were these errors easily avoidable? That’s the question we’ll answer.
Junior League Parity
Some preliminaries: in the aggregate, there’s no significant difference between the three junior hockey leagues. [...] While individual performances vary, the average player who moved from the OHL to the minors or the NHL played as well as the average player who started out in the WHL or QMJHL.
The Significance of Age
Does it matter how old a player is when he puts up big numbers in Junior? Obviously it does – Wayne Gretzky had 70 goals and 112 assists in 64 games for the Sault Ste Marie Greyhounds as a 17-year-old in 1977-78. Seven years later, Dan Hodgson had the exact same statistics when he was a 20-year-old playing for the Prince Albert Raiders. Hodgson was drafted 83rd overall despite his prolific scoring, and had 74 points in a 114-game NHL career. (He is still active in the Swiss National League.) At age 22, Hodgson had 57 NHL points; Gretzky had already scored 1024 points between the NHL and WHA.
So in a qualitative sense, it’s obvious in this case that a 17-year-old player’s performance predicts a much better career than a 20-year-old’s stats. But there is also a strong quantitative relationship between past and future performance. Based on the performance of thousands of drafted players, we can predict how many points a player will score in the NHL when he’s 21-years-old. If he’s 17, four years later, we expect him to score at 72% of his junior rate. But if he’s 20, on average, he’ll retain just 26% of his scoring.
There is a caveat: younger players are a bit less predictable than older players. For a 17-year-old, the middle 50% range of the projection is from 45% to 98%, while for a 20-year-old, it’s from 17% to 33%. This wide range reflects how unpredictable future performance is for NHL players. From age 21 to age 25, Wayne Gretzky scored between 196 and 215 points each season, which is only a 10% variation, while this method predicts a possible 2:1 variation in scoring. The performance of an individual player is much more consistent than it is for the large group of drafted NHL players.
We could narrow the bounds of the projection if we had more data about the players. This method tries to capture a player’s performance despite having no information about linemates, ice time, injury status, size and performance in other seasons. Who you play with can have a profound effect on your performance: Rob Brown played with Mario Lemieux and had 49 goals and 115 points. The Penguins traded him away two years later, and without Lemieux setting him up, he couldn’t crack an NHL roster.
PPG Projections by Age
In the aggregate, players reach their peak performance level at age 22 and hold it for several years. [...] A 17-year-old player with 2 PPG in Junior can expect, on average, to score 1.5 PPG in the NHL at age 22, while an 18-year-old Junior doing the same thing has an NHL projection of 1.0 PPG, which is 40 fewer points over the course of a season. This is the difference between elite players (Joe Sakic, Denis Savard, Dale Hawerchuk) and much lesser players (Jimmy Carson, Terry Yake, Mike Bullard.)
This is very significant for the NHL Entry Draft. An entire year’s worth of players become eligible for the draft, but the players born earlier in the year have a peak value 35% lower than players born late in the year. This is obvious when you consider the difference that one year of physical maturity can make at age 17. In evaluating a player, it is critical to keep in mind his exact age, down to his month of birth.
The whole article, with charts, at Junior Hockey Projections.