> ## Documentation Index
> Fetch the complete documentation index at: https://docs.pullrule.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Ownership Risk

> Understand the risks associated with ownership and how to mitigate them.

Ownership risk highlights how knowledge and responsibility for code are distributed across a team.
Healthy ownership patterns reduce dependency on single individuals, improve resilience, and help prevent “bus factor” risks.

PullRule provides three complementary metrics to measure ownership health:

* **Herfindahl–Hirschman Index (HHI)** – how concentrated ownership is.
* **Ownership spread** – how many effective contributors exist.
* **Coverage overlap** – how often files are touched by multiple people.

## Herfindahl–Hirschman Index (HHI)

The **Herfindahl–Hirschman Index (HHI)** measures the concentration of ownership across contributors.

* **0 → perfectly spread** (ownership is evenly shared).
* **1 → fully dominated by one person** (a single point of failure).

**Lower HHI** = healthier distribution of knowledge.<br />
**Higher HHI** = higher dependency risk.

### Example

* **HHI = 0.10**
  Ownership is well-distributed. Many contributors share responsibility, so the code is resilient.
* **HHI = 0.34** Some concentration exists. A few people own most of the work, but there is still some redundancy.
* **HHI = 0.80** Very high risk. Nearly all ownership sits with one contributor. If they leave or become unavailable,
  the project suffers.

## Ownership spread

The **ownership spread** translates HHI into an “effective number of owners,” calculated as:

```
Ownership spread = 1 ÷ HHI
```

It gives an intuitive sense of *how many people really own the code*, even if the contributor list is longer.

* **Higher values** → knowledge is spread across more contributors.
* **Lower values** → fewer contributors hold most of the ownership (higher bus-factor risk).

### Example

* **Ownership spread = 6.0** Knowledge is spread across the equivalent of six full owners -- resilient.
* **Ownership spread = 2.98** Roughly three effective contributors share ownership. This
  is okay, but still exposed to loss if one leaves.
* **Ownership spread = 1.2** Almost everything is in the hands of a single person. Very risky.

## Coverage overlap

The **coverage overlap** measures the fraction of files touched by at least **two distinct contributors** in a given period.

* **1.00 = every file has multiple contributors** (high resilience).
* **0.00 = all files are owned by just one person** (high risk).

This tells you whether files have redundancy in knowledge or if they risk being siloed.

### Example

* **Coverage overlap = 1.00** Every file has multiple contributors. If someone leaves, others can step in immediately.
* **Coverage overlap = 0.60**
  60% of files have shared ownership. The other 40% are risky, as they depend on a single contributor.
* **Coverage overlap = 0.00**
  All files are owned by exactly one person. The team is at maximum bus-factor risk.

## How to use these metrics

* **Spot weak points:** Look for high HHI, low spread, or low overlap in critical areas of the codebase.
* **Mitigate risks:** Encourage pair programming, code reviews, or rotations to spread ownership.
* **Track over time:** Improvements should show up as lower HHI, higher spread, and higher overlap.

PullRule makes these risks visible so teams can proactively build resilience into their codebase.