AggregateRating
Use ratingValue and reviewCount or ratingCount to summarize real ratings associated with the entity.
Review
Individual Review nodes can include author, reviewRating and reviewBody when those reviews are genuinely published on the page.
Visible evidence
Do not manufacture a 5-star rating solely to obtain a richer snippet. Search engines can treat misleading markup as spam.
Scale
When the rating scale differs from the usual 1–5 range, bestRating and worstRating can describe it.
Product relationship
Place aggregateRating on the Product it actually summarizes, not on an unrelated WebPage.
Counts
Keep reviewCount synchronized with the visible set or aggregate represented on the page.
Validation
Check values, entity relationships and whether rating content is visible to users.
Frequently asked questions
Can structured data always be improved to 100% automatically?
Only when the required data actually exists in the JSON-LD or on the page.
Should every possible Schema.org property be added?
No. A smaller set of accurate properties is better than irrelevant values.