Health Score reflects the proportion of internal URLs crawled by Site Audit that don't have errors. It is a metric that has to be examined within context as it depends on:
The number of internal URLs with errors and
The total number of internal URLs crawled
To understand Health Score, it would be important to understand what issues are.
What are Issues?
Issues are a set of rules or filters which you can apply to all crawled pages for a complete audit of your site. Issues can be an error, warning or notice depending on the deemed level of importance. We term the more critical issues as "errors". These pre-set issues can be configured or new custom issues can be created.
How is Health Score calculated?
Health Score = (1 - (<internal URLs with errors> / <total internal URLs>)) * 100
For example, Site A has 10 pages crawled while 2 out of these 10 pages have errors. Thus, Health Score = (1 - (2/10))*100 = 80%
On the other hand, Site B has 100 pages crawled but 80 of these 100 pages have errors. Thus, Health Score = (1 - 80/100)) *100 = 20%
This also means that while Site A has less pages crawled compared to Site B, due to the fact that the proportion of errors is lower for Site A, its Health Score is higher.
Note: By using the option "Turn off for this project" for "Errors", Health Score will be affected and for "Warnings" or "Notices", it will be not.