Efficiency vs. Expected Value

On its face, this post will come across as confusing.

But I promise, the math is crazy simple. Like really really easy.

Please don't panic.


efficiency goes from -1 to +1.

expected value goes from 0 to 1.

so 0.000 eff = 50% expected value.

and 0.200 eff = 60% likelihood to win the rally.

People think about efficiency usually only in terms of attacking.

But really all we're saying is (won points - lost points) / all points

In attacking, that's (Kills - Errors - Blocks) / Attacks

But it's the same math, for any of the skills.

Take FBSO efficiency. (points won on 1st ball - points lost on 1st ball) / receptions

Maybe your FBSOeff = 0.200

This is loosely translated as "we are hitting 0.200 on 1st ball"

Of course maybe you include reception errors? setting errors? out of rotation? 

Maybe you don't. Doesn't matter, not relevant to this post...

With Expected Value or Expected Points (same-same), all we're doing is using a different, 0 to 1 scale.

Though you do need to know that 0.5 and 50% are the same thing...

But that's the only leap you need to make

0.200 in efficiency is simply converted to 60% likelihood to win the rally.

0.400 efficiency? No problem, that's a 70% chance to win.

0.800 efficiency?!? Wow. 90% chance you're gonna win.

1.000 efficiency, meaning I scored a kill? Boom. 100% chance to win the rally.

hmm, -0.300 efficiency? rough match eh? easy, 35% chance to win

I promise the math isn't scary: 

(Efficiency / 2) + 0.5 = Expected Value

I personally like to think of it as: how far from 50% are you?

Where I always know that 50% = hitting 0.000 in terms of efficiency

Coach says he's hitting 0.280?

No problem, divide it by 2 (.140), add it to 50%

64% likelihood to win the rally.

On this blog, I will skew towards using Expected Value and the 0 to 1 scale.

This is because we operate outside the world of only attacking

and we need a clean way to describe your likelihood to win the rally, at any point.

Saying your block touch put your team in a -0.160 situation is a little tricky to wrap you head around.

Saying you have a 42% chance to win the rally makes it a little easier to digest.

That being said.

I understand and am fully aware that speaking in terms of "Expected Points" is not currently the norm.

The same way the NFL, NBA, Soccer, Hockey all speak in terms of Expected Points and Goals

Hopefully our sport can slowly turn the ship in that direction...