Roll Difficulty

Untold Dawn uses the GURPS 4e 3d6 roll-under
            resolution model. To make a skill check, the engine
            takes your effective skill level (governing stat plus
            training, plus any situational modifier) and rolls
            3d6 against it. A total <= effective level is a
            success; over is a failure.

            A "difficulty" modifier is just a number added to (or
            subtracted from) your effective skill level for that
            one roll. Positive numbers make the task easier,
            negative numbers make it harder.

            # subtopics

            ## The modifier scale

            The GURPS standard difficulty table, anchored at 0:

                +10   Trivial -- automatic for anyone.
                 +6   Very easy -- only a fool fails.
                 +4   Easy -- routine for a trained hand.
                 +2   Favorable conditions.
                  0   Average task.
                 -2   Unfavorable conditions.
                 -4   Hard -- requires real skill.
                 -6   Very hard -- experts only.
                 -8   Dangerous -- the kind of roll people
                       write home about.
                -10   Daunting -- functionally impossible
                       without preparation, equipment, or help.

            ## Worked example

            Your Observation effective level is 12 (PER 13,
            average tier, untrained default of -5 wouldn't apply
            because you're trained to 4 pts, putting you at PER +
            tier offset + tick offset = 12). You search a room
            with two hidden descs:

                hdesc 0     diff 0  -> roll 3d6 vs 12
                hdesc -4    diff -4 -> roll 3d6 vs 8

            On the first you need a 12 or under (~74% chance). On
            the second you need an 8 or under (~26% chance). One
            search command rolls each independently.

            ## Choosing a difficulty for your hdesc

            Pick by how subtle the detail is, NOT by how
            "valuable" you want it to be:

                 +2 .. 0   Things obvious to anyone who
                            stops and looks (the smell of bleach,
                            scuff marks by a door).
                 -2 .. -4  Things you'd only find if you
                            looked carefully (a name scratched
                            inside a drawer, footprints in the
                            dust).
                 -6 .. -8  Things deliberately hidden (a
                            razor taped under a chair, a coded
                            phone number in graffiti).
                -10        Genuinely concealed and likely
                            to need clues to even know where to
                            look. Use sparingly.

            ## Related

            help rdecorate -- the command that uses this
            scale for hidden descriptions.
            help search -- the player-side command that
            rolls against it.