In essence, the validity score is our own internal scoring for reviews.
Each review is checked over by hand to edit out any personal or identifiable information and as we go we mark a review based on evidence submitted, how trustworthy the user is, tenancy dates provided, whether a user has a valid email address and how similar the review is to others.
At the end of this process, we give the review a score which can effectively be looked at as a "trustworthiness" score.
We've found that between 5 - 10 points, is usually something factual. Enough evidence has been provided, the user looks valid and the dates look correct.
3 - 4 points is either a simpler review, less evidence but there might be things you should watch out for in it.
1 - 2 points is a very basic review, it's to be taken with a pinch of salt. These reviews might be contested in future.