Choosing your keywords
A market holds up to 100 keywords, and every average in it is computed across them. The set is the measurement, so choose it deliberately.
There is no absolute position on Amazon. There is only a position on the terms you decided to measure, which makes the keyword set the most consequential choice in a market.
Start wide, then cut
The Explorer makes it cheap to start wide: load a niche, or search your product's ASIN to see every keyword it already ranks on. Then cut. A market is not a collection of every term that mentions your product; it is the set of searches that decide whether it sells.
What belongs in the set
The searches a buyer actually types. Head terms where the demand is, plus the specific variants where you realistically compete. The demand dots in the Explorer's suggestions help: a keyword with very low demand rarely deserves a place in your averages.
Terms where the competition is decided. A keyword you have owned at position 1 for two years teaches you little from day to day. One where three sellers keep trading places tells you something every week.
Your brand terms, if defending them matters. They usually make your averages look excellent, which is worth knowing but not worth being surprised by. Consider giving them a low weight, or a market of their own.
What to leave out
Terms that describe a different product: accessories, other sizes of the category, other uses. The Explorer's automatic weights already mark many of these as Minor, because the products ranking on them differ from the rest of your search.
Near-duplicate phrasings. Two spellings of the same search usually move together, so the second adds little information but doubles that search's weight in your averages.
Use the weights
A hundred keywords is room for a whole niche, and that is where weights come in. Instead of leaving out every secondary term, keep it and give it a lower tier. Keyword tiers and weights explains how.
Reading the set honestly
Coverage is the check on all of this. It reports how often the product was on page 1 across the keywords of the market. A strong average position on nine of forty keywords is a different thing from the same average on thirty-eight, and only coverage tells you which one you are looking at.