# Reliably Detectable Marks

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This section maps to Requirement 1.2-B
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Marks are determined valid and counted by the system if the amount of darker pixels in the bubble area exceeds a configurable threshold. This threshold can be adjusted to have a stricter or looser interpretation of what is considered a valid mark. The recommended and default threshold in the system is 7%, under which 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. 

## Invalid Marks

<figure><img src="/files/b3UVkJPmKNOiN2AnYMND" alt=""><figcaption></figcaption></figure>

## Valid Marks

<figure><img src="/files/I97zoPzJvYHb0SMGhbil" alt=""><figcaption></figcaption></figure>

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