Glossary

Availability — whether a product can currently be purchased. Captured alongside rank because a stock-out produces a rank decay indistinguishable from a demand decline without it.

Basket — a fixed set of comparable products tracked alongside a subject, used to separate market-wide movement from product-specific movement. The single most useful control in this field.

Buy box / featured offer — the default seller on a listing with multiple sellers, capturing most of its sales. Rotates.

Category rank — position within a defined subset of the catalogue. More sensitive than overall rank, and not comparable across categories of different sizes.

Conversion basis — the table or model used to translate rank into estimated units. Always category-specific, marketplace-specific and dated.

Decay — the gradual worsening of rank after a sale, as the recency weighting on that sale diminishes. Produces the sawtooth pattern in low-velocity products.

Featured-position share — the proportion of observations in which a given seller held the buy box, used to attribute a shared listing's volume.

First-party data — your own sales figures. Superior to any rank-derived estimate for your own products.

Gap — a missing observation. Stored as absence, never as zero and never interpolated in the source data.

GTIN — the family of global trade identifiers including UPC, EAN and ISBN. Not unique to a marketplace listing.

Long tail — the deep portion of a catalogue where rank is dominated by decay from individual sales and analysis is largely unavailable.

Noise floor — the range within which a product's rank varies with nothing happening. Established from a quiet period, per product, and the reference against which any movement is judged.

Overall rank — position across the entire catalogue. The workhorse measure, less sensitive than category rank and not gameable by classification.

Pull-forward — demand moved earlier by a promotion rather than created by it. Visible as a post-promotion period below the prior baseline.

Rank type — whether an observation is an overall rank or a category rank, and if the latter, which category. Must be recorded; mixing types invalidates comparison.

Recency weighting — the property that recent sales count for more than older ones in the rank calculation. Undocumented in its specifics; inferred by observation.

Review velocity — reviews accumulated per period, used as a secondary sales proxy. Less noisy than rank for slow products.

Sales rank — position in an ordering by recent sales velocity. A position, not a quantity, and not a measure of revenue, profit or cumulative sales.

Sawtooth — the chart shape produced by separable individual sales: sharp improvement followed by decay, repeated.

Search position — where a product appears for a given query. A distribution rather than a scalar, personalised and regionalised, and a different measurement from sales rank.

Settled level — the rank a product holds after the decay from an event has completed. The relevant measure for evaluating a campaign, as opposed to the peak.

Split-half test — dividing an observation window in two and comparing the summaries. If they disagree substantially, the window is too short.

Survivorship bias — drawing conclusions from a sample selected on the outcome, such as a bestseller list, without a comparison group.

Variant — a size, colour or format grouped under a parent listing. Whether rank aggregates across variants differs by marketplace and category, and it must be checked before comparing.

Terms used loosely elsewhere

Several terms are used inconsistently across the industry and these documents pick one meaning.

"Sales rank" and "best sellers rank" are used interchangeably here and refer to the published position by recent sales velocity.

"Accuracy" in vendor material usually means agreement with their own reference set, not with actual sales. These documents avoid the word without a stated definition.

"Real time" describes a short refresh interval, never live data.

"Organic" means not paid, and it does not mean unaffected by advertising, since advertising's sales feed the systems that produce organic position.

"Market share" derived from rank is an estimate of unit share within an observed subset of a single marketplace, which is a narrower claim than the phrase implies.

"Bestseller" with no category named is a marketing claim rather than a measurement.

Where a document on this site uses one of these, it says which meaning it intends, because the ambiguity is the source of a great deal of confused discussion.