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Call of Cthulhu 7e — meet regular difficulty

R09-regular · Beginner · Component: one unopposed regular-difficulty check

Question and scope

How often does an investigator meet regular difficulty at a supplied skill value? How much does improving that skill by ten points help?

Roll percentile dice, interpreted as 1–100, and succeed when the result is at or below the skill. This report combines all results that meet regular difficulty; it does not mean exactly Regular rather than Hard or Extreme. No success degrees or critical/fumble categories are reported. It excludes bonus/penalty dice, pushed rolls, Luck spending, opposition, and skills 100+.

Inputs

The underlying ordinary threshold component is used only at skills 1–99. The narrower editable range ensures both rows stay within that scope.

Model and runnable source

d(100) represents equally likely results 1–100, not 0–99. .cdf(skill) sums the chances up to and including the skill. type and fail enforce the input contract; if runs its indented instruction when a condition is true. or means any of the listed invalid conditions is enough to stop.

skill = 50
if type(skill) != "int":
    fail("Use an integer skill")
if skill < 1 or skill > 89:
    fail("Use skill 1–89 so skill + 10 remains below 100")

percentile = d(100)
rows = [
    ("Meet regular difficulty at skill " + str(skill), percentile.cdf(skill)),
    ("Meet regular difficulty at skill " + str(skill + 10), percentile.cdf(skill + 10)),
]
output("Alternative skill values: regular difficulty or better", prob_table(rows))

Alternative skill values: regular difficulty or better · Table

Read and independently check

At defaults, the rows are 50% and 60%: exactly fifty and sixty of the hundred faces satisfy the respective thresholds. The improvement is ten percentage points. The die's average remains 50.5, so its mean alone cannot measure this improvement. The rows are alternative skill settings, not exclusive outcomes.

Boundary inputs skill = 1 and skill = 89 produce (1%, 11%) and (89%, 99%). Values 0 and 90 are rejected by this recipe, rather than silently expanding its scope. Equality counts: skill 50 includes rolling 50, not just 1–49.

Adaptations and composition limits

For a fixed skill 1–99 without the improvement comparison, use the single query in L05. Degrees of success need additional thresholds and category boundaries; bonus dice are not two independent d100 rolls because they share a units digit. Neither extension is implemented here. Do not treat the two table rows as consecutive attempts or a pushed-roll policy.

Rules and related lessons

Chaosium's Call of Cthulhu 7th Edition Quick-Start Rules, Reading D100 (Percentile Dice) and Skill Rolls and Difficulty Levels (p. 10 of the linked PDF), source checked 2026-09-15. Rules are paraphrased. Independent human rules review and browser/novice review remain pending.

L05 introduces inclusive roll-under; L07 explains the table structure; L08 builds a related comparison with a loop.

Content ID: R09-regular · Review status: pilot

Prerequisites: L05, L07, L08

Rules review: source checked 2026-09-15; human review pending