> For the complete documentation index, see [llms.txt](https://docs.voting.works/vxsuite-tdp-v4/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.voting.works/vxsuite-tdp-v4/system-performance-and-specifications/reliably-detectable-marks.md).

# Reliably Detectable Marks

Marks are determined valid and counted by the system if the amount of darker pixels in the bubble area exceeds a configurable "definite" mark threshold set in the [Election Package](/vxsuite-tdp-v4/readme/election-package.md) system settings file. This mark threshold can be adjusted to have a stricter or looser interpretation of what is considered a valid mark. On both VxScan and VxCentralScan, the voting system does not process any marks as ambiguous. All marks are either valid or invalid.

The recommended and default threshold is 7%. With this threshold setting, hand marked paper ballot interpretation detects a valid mark per the mark conditions described by VVSG 1.1.6-H.

At this default threshold, marks such as a light dot, a fold through a bubble, a stray mark outside of the bubble, or a small line just on the corner of a bubble will be considered invalid. Marks such as a completely filled bubble, half filled bubble, or an X in a bubble will be considered valid.

Examples of invalid and valid marks along with the score for that mark (in green) are shown below.

The mark threshold set on a scanner can be checked by an election official at any point by viewing the readiness report for that device as described in [Diagnostics](/vxsuite-tdp-v4/readme/diagnostics.md) (see also [VxScan Diagnostics](https://docs.voting.works/vxsuite-user-manual-v4/vxscan/vxscan-diagnostics) & [VxCentralScan Diagnostics](https://docs.voting.works/vxsuite-user-manual-v4/vxcentralscan/vxcentralscan-diagnostics) in the user manual).

### Invalid Marks

<figure><img src="/files/KPbTG26yC2ouh8Zm5Axz" alt=""><figcaption></figcaption></figure>

### Valid Marks

<figure><img src="/files/lycAx3JFNlIrbidOBtCb" alt=""><figcaption></figcaption></figure>

### Marginal Marks

The system also supports setting a marginal mark threshold for flagging ambiguous marks close to but not quite over the definite mark threshold. If a marginal mark threshold is set and marginal mark adjudication is enabled, the definite mark threshold can be increased without having to worry about completely missing marks.


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