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Tell me your birthdate, and I’ll tell you your future

This is not an astrology post.

E. Wilson via Wikimedia

Horoscopes and astrology have enjoyed a bit of a renaissance lately, and while this article is not about NHL players’ astrological signs... well it is in a way, but I’ll get to that in a moment. Astrology is a narrative seeking to make clockwork order out of a complex universe. It’s about writing out the gods of fortune and substituting a more structured and predictable story arc about who we are, and why things happen to us.

I think reading your horoscope every day is a lot like reading a self-help book. It’s a handy way to have an internal dialogue with very basic ideas about you and your life choices. Largely harmless, imaginary in its concepts, but real in its use. It doesn’t predict your future though.

If you were to take a large group of people, say around 1,000 of them, ask their signs and you found a really strange skew in the answers about how that mapped onto where they work and how successful they are, you could decide that being a Scorpio is different from a Cancer and both are different from a Taurus. Or you could ask if something more difficult to pin down is going on.

What Sign Are You?

Let’s ask the NHL what sign they are. Quant Hockey and Elite Prospects Premium have some charts available I’m going to use. Here’s the NHL in 2021-2022 by birth month:

Quant Hockey

The total they are using is 1,123 which includes regular roster players and all the one-game callups. That result looks weird if your expectation is that each month should be fairly even. To see if that expectation is reasonable, I spent a few minutes on Statistics Canada’s site. Not all NHL players are Canadian, but they are still the biggest single cohort:

This is the years available that cover the birthdates of most of the players in the NHL today. And from this, what we should perhaps expect is a slight decline in our NHL results in the winter months, with January and the short month of February having low numbers similar to November and December. There is no skew towards the start of the year like in the NHL this season.

An entire country’s births over many years is different to one season of the NHL, so maybe that NHL season is just weird by chance. To check that, I averaged seven NHL seasons from 2015 to this year. I carefully chose that number of seasons by stopping data entry when I got bored. This is not rigorous science here, but an examination of a phenomenon that has been studied more rigorously by others.

The average gives a very clear picture that 2022 isn’t all that weird. One other thought I had was that over those years form 2015 to 2022, that’s mostly the same people year after year being counted. To check that, you can look at any year you like on Quant Hockey and discover this same phenomenon in the 2000s, the 1990s and the 1980s. It disappears when you get back to the 70s, 60s etc. Which is not something I have a firm explanation for, but it’s interesting and tends to show there’s nothing inevitable about this modern skew.

Draft Cutoff Date

Whenever I talk about birth month bias, the topic of the draft birthdate rules muddies the waters to deep dark brown. So let me wade into the mud for a bit.

The cutoff for the NHL draft is September 15, so this year’s draft is for players born beginning on September 16, 2003 up to September 15, 2004. There’s always some players born before the earlier date, but not many. The NHL draft traditionally shows the same skew as the NHL itself, and the relative success of players compared to their draft position shows that later birthdate players tend to be drafted “too low”. This is the narrative that tries to use that cutoff date as the explanation for that draft analysis:

People think that because the players born between September 15 and the end of December are actually the oldest players in the draft, their junior stats are naturally impressive, so scouts and GMs discount them and don’t draft them as high. The players born in the summer are the youngest players, and they are impressive because they’re playing well very young. Therefore players born in the final quarter of the year are drafted too low.

Here’s why this cannot be the answer: A player born in December is a few weeks different in age to one in January at the draft. Why the big difference then?

Why isn’t there a big step down from August to September to October delineating the cutoff date? There’s usually just the same gradual decline from January to December.

Why isn’t the summer quarter a big jump up if they’re assumed to be younger guys succeeding against the odds?

Even if the cutoff date affects scouting opinions of players and affects draft position, why isn’t actual talent flattening out the NHL skew by the time players have been in the league for a while? It’s not like a player born in November is never drafted! That other draft bias can only account for some of this — the one thoroughly studied in the NBA that shows that higher draft picks get more games played.

The draft cutoff date narrative cannot explain the actual draft results or the NHL demographics. It just seems like it should be the answer, particularly if you focus only on the final quarter of the year, where players are most underrated at the draft and most underrepresented in the NHL.

When you remember that the phenomenon begins in the early spring and summer months, not October, you need to claim that players born in the summer are downranked because they’re younger and players born in the fall are downranked because they’re older, and when anyone says but December to January, though, you have to slap your hands over your ears and sing la la la la la.

Does the cutoff date and the age difference affect who ends up in the NHL long term? Likely a little if late birthdate players have to work a little harder to get into the NHL in the first place. However, the idea that this persistent phenomenon is just caused by the draft date crashes fatally on the rocks when you look at these charts from Elite Prospects for this season:

Your Lane is Picked For You

It really looks like this phenomenon, while international, is worse in the junior leagues than in the NHL. To know for sure would need rigorous analysis, but the NHL seems to be somewhat ironing out this bias, and it is a bias. There is no reasonable explanation for why this skew persists across hockey leagues and over decades. It’s not just hockey either, you can find this birth month skew in many other fields, in educational evaluation and other forms of evaluation. What is in common in the areas where it’s discovered is that people get together and assess applicants and choose who they think is best. So unless you think being an Aquarius makes you better or that half the Capricorns are bad while the other half are good, something else is going on.

Research into sports, education and other areas of human endeavour suggests that the culprit is childhood growth and development, and that in a given grade in school or a junior hockey team, both organized by birth year, the older, more mature and bigger kids prosper and the little ones born in the later months don’t. This theory fits with the general decline from January to December, and makes a lot of sense for both academic and physical pursuits.

The system corrects itself somewhat as players are drafted — and here is where the draft cutoff date might actually be more properly seen as a corrective, not causal. But examinations of the draft and player performance show that birth month gets you drafted lower than you “should have been”. But if you do get drafted, talent will out to some extent, even if that January baby drafted in the first round gets his 10 NHL games, and you have to toil in the AHL and prove yourself.

Market Inefficiency!

So who cares, though? Well, one thing is that players who aren’t drafted are going to tend to be those late birthdays. Worse is that some small, immature players will get pushed out of elite hockey as children and end up never seeing the NHL. But the NHL can’t fix that, only an overhaul of junior hockey systems can do that and they should tackle racism first and hardest. But biases can be exploited, and a smart team might see players born outside the first quarter of the year as more likely to be hidden gems and better than everyone else has been led to expect. Those players are cheaper to acquire in many ways — trade cost, draft pick, or they might not cost anything at all:

  • Alex Steeves - undrafted FA - born in December
  • Joe Duszak - undrafted FA - July
  • Bobby McMann - undrafted FA - June
  • Nick Abruzzese - 4th round pick - June
  • Pontus Holmberg - 6th round pick - March
  • Michael Bunting - 4th round pick - September
  • Ondřej Kaše - 7th round pick - November
  • Ilya Mikheyev - undrafted FA - October
  • David Kämpf - undrafted FA - January (hey, it’s not a perfect predictor of underrating)
  • Pierre Engvall - 7th round pick - May
  • Ilya Lyubushkin - undrafted FA - April
  • Mark Giordano - undrafted FA - October

The NHL is not in the age of Aquarius. When you’re a team looking to find gold in every nook and cranny, this is the age of Sagittarius.

The longer form look at the research on this topic is here:

My theory on why this happens in the NHL has changed over the years since I wrote that. I wasn’t giving enough weight to the fact that junior players are ranked for the NHL draft years before they’re 18. That ranking is taking place, in effect, when they are pre-teens as they are drafted into Canadian junior hockey or moved from one club to a better one in Europe. The pool of players in elite hockey is huge, and it has to be culled to something manageable long before team scouts start making lists for draft day. Once a player gets marked out as having potential — or not — that belief will persist in the face of a lot of contrary evidence. Your lane is picked for you.

Maybe this phenomenon disappears in the pre-80s years because the scouting and prospects industry hadn’t been invented yet, and junior hockey teams were made up out of players who lived near the team, not out of competitions to land the elite players and win championships to make money on playoff tickets. But that’s just a guess.

If you want better NHL players, though, the answer seems obvious: Don’t make your mind up about people when they’re children. Let them show you who they are in their own time.