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Seems like people are constantly misunderstanding aggression factor around here. Hopefully this can clear some stuff up.
What is AF
If you look at what aggression factor is in PokerTracker you'll see that it's:
(Raise % + Bet %)
----------------
(Call %)
It's the ratio of the amount of time that a player is aggressive vs the amount of time that he's passive. It's a pretty simple concept but it's easy to misunderstand and misapply.
How to use AF?
Aggression factor is a unit-less ratio. It's not that useful without some other information. Just like most other things in poker, it's almost always situation and player dependent, the only exception being a player with an AF of 0.
For example, if someone's aggression factor is 0 over 100 hands we can assume he's pretty passive. He's never bet or raised, only called and folded. If he WERE to suddenly raise us we'd be scared.
Now let's see how AF affects our everyday decisions.
Take 2 players. A 50VPIP and an 18VPIP. Think about the approximate handranges that they're seeing a flop with.
50VPIP (fish):
{22+,A2s+,K2s+,Q2s+,J4s+,T6s+,96s+,86s+,76s,65s,A2o+,K5o+,Q7o+,J7o+,T8o+,98o}
18VPIP (TAG):
{22+,A5s+,K9s+,Q9s+,J9s+,ATo+,KJo+,QJo}
Now think for a second about how each of those hand ranges hits the flop. The TAG is going to be making a lot of top pair, good kicker hands, as well as sets and combo draws. The fish is going to be making a lot of top pair bad kicker and midpair type hands.
Say these players have a flop distribution (just guessing here)
Fish
----
10% top pair
10% middle pair
10% bottom pair
8% flush draw
1% set
1% combo draw
60% air
TAG
----
20% top pair
8% mid pair
2% bottom pair
8% flush draw
1% set
1% combo draw
60% air
Let's look at what happens when you bet into them
TAG
----
Raise: Combo draw (1%), set(1%), flush draw (8%), top pair (20%) = 30%
Call: Bottom pair (2%), mid pair (8%) = 10%
Fold: Air
Fish
----
Raise: Combo draw (1%), set(1%), flush draw (8%), top pair (10%), mid pair (10%) = 30%
Call: Bottom pair (10%) = 10%
Fold: Air
These 2 players have the exact same AF (3) but you can see that the fish is way more "aggressive." His range when he raises you on the flop is weaker than the TAGs because his starting hand range is weaker.
Limitations of AF
If you look at the AF forumla, you'll realize that it doesn't include fold %. This is a pretty big limitation of AF and one that people seem to forget about. Let's change the above example a little.
TAG
----
Raise: Combo draw (1%), set(1%), flush draw (8%), top pair (14%) = 24%
Call: Some mid pair (2%), some top pair (6%) = 8%
Fold: Bottom pair (2%), some mid pair (6%), Air (60%)
Fish
----
Raise: Combo draw (1%), set(1%), flush draw (8%), top pair (10%), mid pair (10%) = 30%
Call: Bottom pair (10%) = 10%
Fold: Air
The fish's actions haven't changed, but we've had the TAG fold some of his weaker made hands and slow down a little with his top pair hands. This is probably closer to a real distribution. Notice that AF is STILL 3! Even though we've strengthened his distribution for raising he still has the same aggression factor. This is where other stats on your HUD are going to be useful. Things like "Call continuation bet %" and "Fold to continuation bet %" will tell you whether a player with a high AF is playing raise/fold with his good hands or he's a player who bluffs with air a lot.
Summary
Don't look at a player with an AF of 0.8 and a VPIP of 80 and say "durrr better fold to his flop raise, this guy is passive!"
