# 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

| Input | Default | Supported values |
|---|---:|---|
| `skill` | 50 | Integer 1–89, leaving room for the +10 comparison below 100 |

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.

```dice
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))
```

## 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](../tutorial/05-roll-under.html). 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](https://www.chaosium.com/cthulhu-quickstart/),
**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](../tutorial/05-roll-under.html) introduces inclusive roll-under;
[L07](../tutorial/07-probability-tables.html) explains the table structure;
[L08](../tutorial/08-comparison-loops.html) builds a related comparison with a loop.