If we jam, we're only going to get called by better.
Let's play around with the numbers and make some assumptions. We'll assume villain is either on a FD or a set.
set=?; (* percentage he has set *)
fd=?; (* percentage he has FD *)
set*(-120)+fd*(120) (* EV calc for jam *)
fd*(.18(-120)+.82(120)) (* EV calc for c/f *)
So EV calc for jam should be self explanatory, if he has a set we're losing 120, and if he has a FD we'll assume he folds to our jam and we win the 120.
In the EV calc for the c/f, we're saying whatever percentage of the time he has a FD, 18% of the time ( 9 outs * 2 (rule of 4 and 2) ), we're going to lose 120, and the other 82% of the time, we'll win what's in the pot. There's some variations we can do this calculation if we want to turn our action into a check/call instead of a check/fold, but let's keep it simple and get a ball park idea of what play is better.
I'm going to plot two equations and get an idea of what happens when we let the probability of a set go from 10% to 90%.
Mathematica source code:
f[x_] := x*(-120) + (1 - x)*(120) (*EV calc for jam*)
g[x_] := (1 - x)*(.18 (-120) + .82 (120)) (*EV calc for c/f*)
Plot[{f[x], g[x]}, {x, .1, .9}, PlotLegends -> "Expressions",
AxesLabel -> {Chance of Set, EV}]
Result:
blue line is the jam, yellow line is the c/f.
Folding out the 18% equity the FD has when we jam isn't worth how much we lose when our opponent has a set.
If we check/fold, we're always winning money short of our opponent having a set 100% of the time, and the more likely our opponent has a FD, the more money we win. Because the FD has only a harmless 18% equity, we don't need to be so aggressive to be profitable, and we see how damaging jamming can be if the opponent does have a set.
We already winning a bunch of money most of the time when the player is on a FD, and the set is so damaging to a our jam, c/fing is the logical choice for unknown players.