This only answers...what it shouldn't be (and it's not a One-And-Done roll, either. It's the narrative the game master invokes with every npc, context clues, scrutiny and awareness rolls, inquiry and interrogation, application of skills and delivery and critical thinking skills...sometimes). How else would you quantify the investigation nature of the game besides the fellowship based skills, investigaiton skills, disposition tables and modifiers, subtlety track and other elements of the game CPS and the bandwagon gloss over?
This is what I keep wondering when I hear the "one and done investigation" argument, but I've never been able to articulate it as well as this.
I'm not sure what you mean by "quantifying" an investigation. I honestly have found the best way to run an investigation is to actually run the matter to be investigated with yourself in your head, and maybe roll a few dice where appropriate. Then you proceed step by step, make note of who knows what and what other parties get involved until someone calls the inquisition or the inquisition notices on their own. It's fairly straightforward and doesn't need many rules beyond a somewhat sane skill system. Abstracting and keeping track of dispositions is an unnecessary step you can simply omit. You are far better off making short cliff notes of the interaction instead. It allows you a clearer memory of what occurred later on, as well as a more organic portrayal of NPCs. Abstracts tend to inadvertedly lead to categories, where everyone with disposition X is the same. It's a counter-intuitive approach. The same is true for subtlety. Every environment is different. Instead of using an abstract score, clearly define the environment instead.
I tend to use something like this checklist:
1. What is the theme of the world?
2. What ecological factors of significance are there, if any?
3. What is the current political situation?
4. What is the current economical situation?
5. What general laws exist?
6. What customs exists?
7. What is the average citizen's disposition, factoring in the above?
8. What is the ruling class's disposition?
9. How competent are the authorities?
10. Does organised crime exist? And how?
11. What is the "threat" and how does it tie into the above?
12. Who are prominent personalities? Quick list of name, three positive and negative traits each as well as function.
13. Who are the PCs likely to meet? Jot down three merchants, three police, three [insert important faction on planet here] as above.
14. If one or more of your PCs are from this environment, make sure to needle them about what they envision it as to prevent SNAFU. Why is this at the end? Simple: It's easier to have setting discussions when you have your notes intact and present.
I also, rarely, roll in investigations. Generally rolls come into play for forensics, hacking and other secondary avenues of investigation, while the face to face interaction is something I generally RP. In some cases, like suspect interrogations, I may have the PCs roll to see if they can make him or her crack, but if they've done their job correctly, wether he does or not should not matter. 40k is even easy mode there, because you can literally rip knowledge from a dead man's brain. I'll also note that social skills can and should use "assists", even from folks without the skill trained. I recently had a prisoner in a cell with three sororitas and a redemptionist cleric. I gave the one asking the questions a +30 for the roll, because that much witch burner in your face is -scary-, wether they're trying to be or not.
As for the dicerolling side, ergo searching for clues, it amounts to awareness tests per person, per room, and may have perception based follow-ups on a variety of tradeskills and secondary skills that sadly no longer exist in DH2e. If someone is excessively thorough, such as methodically scouring every inch of a room, they may even find things you simply cannot plausibly miss even on a failure (though more subtle stuff may go unnoticed of course). Furthermore, my players are free to roll an Int+skil check at any time to ask if what they can plausibly extrapolate from a situation. For example, Int+Wrangling was used to assess how a guard dog would react and likely be trained.
You can put a lot of dicework into investigations, without actually resorting to something so horrific as the subtlety track that lets players game things in an abstract manner that makes me cringe at the table. I'm personally really glad my players only do that when we're in "timeskip" mode, because if someone was like "I burn X influence to decrease our subtlety score" instead of "I walk into the bearau of investigation disguised as a janitor with a murder cogitator stuffed in my cleaning utensils." I'd feel I was doing something horribly wrong as a GM. It also leaves me hanging entirely as a storyteller. Basically, it's assumed the inquisition "somehow" erased the tracks. Great. I'm oh so not impressed by your complete reliance on apparently more competent agents, especially since I don't know what the player has done until he tells me, and a system like that kind of encourages not even thinking about the steps necessary to disappear in the shadows or fabricate a false identity.
TLDR: Create an environment. Be the criminal for a bit. Take notes on the crime. Then run the game.
Edited by DeathByGrotz