Wordmetrics performs best on search topics which return deep text-filled pages.
If your search term is a product name or a product category, your search results will probably be shopping pages that have very little text. In these cases, the generated Semantic Index may be poorer quality.
Here's an example web page returned from the first page of Google search results for the search term "Buy cheap trainers":
While this website has done an excellent job of arriving on page one of search, it's probably not because of their landing page text content, which is very light. So a search term like "buy cheap trainers" will likely result in a non-representative Semantic Index.
What search terms work better?
If you need to drive product or product-category related traffic, try to choose a search is topically relevant to the above search term, but more informational. For example: "How to prevent runner's knee?"
This page will return important contextual information which can be used as part of Wordmetrics statistical analysis.
Summary: When choosing search terms for contextual optimization, it is recommended that you choose informational topics and not simple product names or categories.