DealIndex Methodology v1.0

What we track

DealIndex continuously monitors UK retailer prices across tracked products. For every product we record:

• RRP — the retailer’s declared recommended retail price at each observation.
• Sale price — the price the retailer actually charges, inclusive of promotions.
• Observation timestamps — at least once per day per product; more often during retailer sale events.

We hold a rolling 90-day price history per product per retailer for discount verification, and a 180-day RRP observation window for claim verification.

How we define a “real” price drop

A sale is a real price drop if and only if all three of the following hold:

1. Stable-with-tolerance RRP. The retailer’s declared RRP has varied by no more than ±2% (coefficient of variation ≤ 0.02) over the prior 60 days.
2. Actually-charged. In the prior 60 days, the retailer actually transacted within 5% of RRP on at least one day.
3. Beats the 90-day floor. The sale price is at or below the 90-day low for that product at that retailer.

Basis: CTSI Pricing Practices Guide (2016); CMA 2024 reference-price guidance.

How we flag a “fake” deal

A sale is flagged fake if any of the following is true:

• Inflated-advertised. The advertised discount is ≥2× the real saving versus the 90-day median.
• Pre-sale RRP spike. RRP was raised by ≥10% within 30 days before the sale began.
• Never-charged RRP. The retailer did not actually charge within ±2% of the claimed RRP on any day in the prior 180 days.

Basis: ASA/CAP RRP guidance; CMA DMCCA enforcement drive (Nov 2025); Which? 2024 Black Friday investigation.

Retailer quality score

quality = 0.30 · avg_genuine_discount       (capped at 95th percentile)
        + 0.25 · share_beat_90d_low
        + 0.15 · genuine_discount_frequency
        + 0.10 · price_stability
        − 0.35 · fake_deal_rate

• Retailers with fewer than 20 tracked deals: “insufficient sample”, not ranked.
• avg_genuine_discount capped at 95th percentile across eligible retailers.

Seasonality rule (year-1 protocol)

• Amazon-sold ASINs: Keepa 3-year price history. Citation: “Source: Keepa, 3y window”.
• Non-Amazon SKUs: ≥2 external sources in agreement required.
• Hard floor: no seasonality claim with less than 2 years of combined data.
• Labelling: year-1 claims carry “Seasonal signal — external data” tag.

Right-of-reply

Every named retailer receives their data 48 hours before publication. Corrections accepted by [email protected] until 18:00 BST the day before publication.

Forecast accountability

Every monthly report’s watchlist is scored in the following month: hit rate, average forecast error, and named misses. We do not silently retire failed forecasts.