What determines the strength of radroaches and other incidents?

After playing for a while, I notice that the radroaches are getting signifigantly tougher. At first, two low-level dwellers could easily take care of a couple of roaches with their fists, without taking any significant damage. Now I have 4-5 level 20+ dwellers with trained stats and laser pistols, and they often end up at near 50% health after an infestation. Fires seem to scale in a similar way (but aren't really as bad in my experience).

I think the following things are likely the causes, but I haven't been able to confirm this:

• Room size
• Room upgrade level

Some other factors that may be taken into account (but I consider less likely) include:

• Amount of dwellers in the vault
• Room type
• Dwellers' levels
• Time played

What is causing the radroaches to become so powerful?

Speaking from personal experience, I haven't noticed a general trend in radroach attacks becoming more powerful. Even with a vault of 130 dwellers, I've still encountered radroaches in groups of size 2-8, so I believe it's random.

However, I believe that the maximum size of the radroach infestation depends on the room it happens in. I've never had roaches spawn inside a fully-staffed room and present a problem. I have, however, had issues when a large radroach wave spawned inside an understaffed room (e.g. 2 of a possible 6 dwellers) or when dwellers were on low health from a recent raider/deathclaw attack.

My strategy for dealing with incidents that spawn in the vault is to keep rooms either empty or full, but not in between. If an incident spawns in an empty room, it'll spread until it reaches a staffed room. If the incident is roaches or molerats, this thins their numbers and makes short work of the problem. If it's a fire it's just a matter of letting it die out. All of my dwellers are equipped with weapons that do at least 4 damage, so if an infestation spawns in a staffed room they deal with it trivially.

It may also depend on the dwellers' endurance, which is thought to decrease incoming damage. If that's the case, a combination of level and endurance serve to make dwellers much more survivable. Personally, I send low-level dwellers into the wasteland with my best weapon to boost their level rapidly.

From experience (not testing), I believe it's based off of average level/health of the specific room and room size.

When levels are mixed, lower level dwellers appear to take more damage than when the levels are more or less equal. If it were not dependent on the room itself, rooms full of level 1's would get crushed and they don't seem to.

It won't be clear whether it's levels, health, or endurance stats that are affecting the severity. This is because dwellers will gain health and they level, based on their endurance.