The Resistance Table

In Call of Cthulhu, there was a mechanic called the Resistance Roll, to be used when two characteristics were pitted against each other. It could represent shoving someone out of the way, STR vs SIZ, or an Investigator dying from poison, CON vs POT (the potency of the poison). A simple but clunky formula gave you the chance of success given the characteristics, but there was also a table that had the math already done for you for characteristics from 1 to 31 against characteristics also 1 through 31. The seventh edition of CoC did away with this mechanic, as did Delta Green, which replaced it with the opposed roll. So why do we care about this defunct mechanic? Lots of folks like to use CoC content in their DG games (including old DG content published for CoC) and the Resistance Roll is a tricky mechanic to convert. This post seeks to compare it to DG's opposed rolls and offer some conversion advice.

The Resistance Table

The horizontal X axis represents the passive characteristic, while the vertical axis represents the active characteristic. So if you wanted to see what chance an Investigator with 10 STR would have in an arm wrestling contest against a deep one hybrid with 15 STR, you would start at the row marked 10 (or 50) and follow over until you got to the column for 15 (or 75). Only a 25% chance of success.

The active characteristic is the one initiating the contest. For example: the cultist's POW when they're trying to subjugate someone with a magic spell. The passive characteristic is the one that's resisting the active characteristic. If an Agent is using Unarmed Combat to disarm a knife-wielding gangster, the gangster's Melee Weapons fighting back roll to stab them is the passive characteristic.

Now Delta Green has two different ways of doing opposed rolls. The generous method, allows for success even if you roll over your skill or STATx5 so long as your opponent also fails and rolls higher than you. The strict method requires you to roll under or equal to your skill or STATx5 in order to succeed (and beat your opponent's roll if applicable). This is a table for the active characteristic's generous chance of success.

Opposed Rolls (Generous Method)

 And here's the table for if the strict method is used.

Opposed Rolls (Strict Method)

You might've noticed that the axes aren't perfectly smooth. This is because opposed rolls aren't just for stats in Delta Green, they're for skills too. Rather than unnecessarily double the amount of tables (one for skills, one for stats), I combined the two into one chart. The axis goes from 5% to 99% in a skill and 1 to 100 in a stat. Because the DG system treats STATx5 tests for stats over 19 a little differently, there's a 99 sandwiched between 95 and 100 (which is really just a STATx5 test for a stat at 20). The scale of the axis also changes past 99. If I kept to the increments of 5, I would need a column and row for every stat from 20 to 100. So I switched to increments of 25, still increments of 5 on the scale of ability scores.

First, lets compare the Resistance Table to DG's generous method of opposed rolls. The red shows where the Resistance Roll favors the active characteristic compared to an opposed roll. The blue shows where an opposed roll would give the active characteristic a better chance of success compared to the Resistance Roll. The second image is just the absolute value of the difference from the Resistance Roll. The more purple the cell, the greater the difference is.

Resistance Roll vs Generous Opposed Rolls

Absolute Value of the Above

The top left quadrant of the chart shows us how humans fare against other humans. There's a reddish smear where medium to higher active characteristics are matched up against low passive characteristics. In this area, an active characteristic has a better chance of success with a Resistance Roll in CoC than an opposed roll in DG. In plain English, CoC favors competent (but not infallible) opponents trying to 'mop up,' weaker ones when compared to DG. There's also a corresponding blueish smear where lower active characteristics are matched up with medium to higher passive characteristics. To translate into plain English again, DG favors weaker opponents trying to overcome competent foes. 

When it comes to humans trying to overpower inhuman forces, DG Agents have a better chance of success than CoC Investigators. Vice versa, DG's monsters will have a harder time overcoming Agents than CoC's monsters will Investigators (and that's even before you take into account that Agents tend to have higher stats and more skill points than Investigators). I wasn't expecting this, but DG seems to favor the underdog more than CoC does.

Comparing the strict method to the Resistance Roll gives us a similar picture. The difference between the two resolution mechanics is less 'spiky,' and the chart highlights DG's tendency towards "ineffectual slap-fests" when both parties are incompetent or weak.

Resistance Roll vs Strict Opposed Rolls

Absolute Value of the Above

Overall, I'd recommend using the generous method when converting Resistance Rolls, as it more closely matches the binary pass-fail outcomes of CoC than the ternary outcomes of the strict method where both parties can fail. The biggest difference you'll find is that CoC's resistance table has lots of match-ups where success or failure is guaranteed. When one characteristic is 10+ points over another one, success or failure is automatic, no rolling required. For a practical example, there's an NPC in Eyes Only with a pair of magic staves that give him 5D20 extra points of POW. It's only for the purposes of the Resistance Table but because he already has 23 POW, even the lowest possible roll still puts him 10 points above even the most POWerful Investigator. There was no way to resist his spells. For a less complicated example from the same book, one of the scenarios has an artifact that contaminates you if it beats you in a POW vs POW roll. In CoC, its POW of 30 meant that touching it spelled certain doom. In Delta Green, Agents could have up to a ~23% chance to escape harm!

For the TL;DR: in most cases where a CoC module would call for a Resistance Roll, using opposed rolls will only slightly improve an Agent's chances or give them a chance where there was none at all. Use the generous or the strict method depending on whether you want two or three possible outcomes. It doesn't give the exact same results, but I don't think the changes are bad.

Unfortunately, this doesn't help us at all with converting old CoC poisons into Delta Green because DG uses a completely different system for poisons and venoms. Fortunately, Delta Green's poison system (more accurately a 'toxin system') produces similar results to Call of Cthulhu. Because poisons in CoC do damage equal to their POT if they overcome an Investigator's CON, and half that if they fail, they either kill you dead or remove a large chunk of your HP. Just like poisons in DG! My best advice is to use the table in the Agent's Handbook to convert mundane poisons. Any supernatural or alien poisons under 12 POT should be a flat roll of damage anywhere from 1D4 for 1 POT to 2D8 for 11 POT. A CONx5 roll should be allowed to reduce the damage, just like for a poison's failed Lethality roll. For 12 POT poisons and above, start with 5% Lethality, add 5% for every additional point of Potency, and adjust to taste.

Some poisons in Call of Cthulhu have effects other than damage. For those, just have the Agents roll CONx5 to resist the effects. If the poison has a POT of about 20, apply a -20% modifier to the roll, -40% if it has about a POT of 30 or more.

This is the code for the program that calculates the chances of an opposed roll. You can use it to find probabilities of match-ups that aren't already in one of the provided tables. If you don't have Python, you can always copy-paste the code into an online compiler and run it there. I tried to design it to be user friendly, so don't be afraid to give it a shot if you're not a coding whiz.