How is the online score calculated?

The implementation matches the design doc here: Storage Node Downtime Tracking with Audits.

In production, we have 12-hour windows and a 30-day tracking period, which translates to 60 windows per tracking period and two windows a day. Every single audit the storage node gets will affect its online score to some extent. For example, if a node got audited during 30 seconds of downtime, that offline audit will have a negative effect on the online_score of the storage node. But other audits that happened inside the same 12 hour window will be equally weighted.

So in one 12-hour window, if a storage node gets 1 offline audit and 10 total audits, the online_score for that window will be 0.9. Then, the score for that window will be averaged with all the other windows in the 30-day tracking period to calculate the storage node's overall online_score. So if this storage node had perfect uptime outside of the 12-hour window mentioned above, the online score would be approximately

(59 * 1.0 + 1 * 0.9)/60 = 0.99833

The online_score is reported back to nodes not immediately but with some delay (up to 12 hours), so it may not drop until long after the downtime happened.

You may see a graphical explanation on the forum:

You may use these scripts to find out when your node was offline.

For a more detailed description of the downtime tracking calculations, please refer to this blueprint.