The GPK25 Index

Methodology

Constituents. 25 fixed constituents; each is one exact card at one exact grade from one grader (PSA). Five flagship OS1 cards plus Adam Bomb 8a PSA 9 are permanent anchors; the rest were selected by trailing-12-month dollar volume within segment quotas (eligibility: at least 12 recorded sales at the benchmark grade in the trailing 12 months; quota slots a segment cannot fill shift to the OS1 field pool, then OS2).

Live value vs index price. The Card value column is the live value of that exact card — identical to what its own page serves, recomputed continuously from recorded sales with recency weighting. The 90d median column is what the INDEX uses, and it is deliberately a different statistic: an unweighted median over a fixed 90-day window, so a single hot sale cannot jerk the index. The index LEVEL is a chained monthly close and moves only on monthly prints; the card values beside it are live. If those two columns ever contradict a card’s own page, that is a defect — csp-gpk25-guard.timer fetches all 25 card pages every 30 minutes and fails loudly if any of them disagrees.

Pricing. A constituent's value for month M is the median of its recorded sales in the 90 days ending the last day of M (dates US/Eastern). Fewer than 3 sales: the last median is carried forward and the month is flagged stale. Sales above 5x or below 0.2x the trailing median are excluded as outliers, along with sales our pipeline flags as suspect. Real sales only — never estimates.

Index math. Equal-weighted and chain-linked: each month's return is the simple average of constituent month-over-month changes (carried values count), and the level compounds from 1000 at inception (2024-08, the first month where at least 20 of 25 constituents had fresh medians). Constituent swaps are spliced through the chain — the outgoing card's final month still counts and the incoming card starts the next full month — so list changes never jump the level.

History & maintenance. Months before 2026-07 are rebuilt using today's card list (shown dashed), which makes the past look better than a live index would have: today's list is chosen by today's trading volume. The current month is recomputed daily as provisional (dotted) and is never chained until it closes. The list is reconstituted every Jan 1 (first: 2027): anchors never leave; a non-anchor is replaced only after logging fewer than 12 sales in both Q3 and Q4, at most 5 swaps per year. Mid-year, the only allowed changes are an automatic swap after 6 consecutive stale months and data corrections. Current segment quotas (method v3, re-weighted 2026-07 to measured liquidity): 6 OS1 keys, 7 OS1 field, 2 OS2, 3 OS3, 5 OS4/OS5, 2 premium (PSA 9/10). Anchors are exempt from volume ranking but still subject to the eligibility bar. Nothing about the list is discretionary or random: every non-anchor seat is earned by measured trailing-12-month dollar volume within its era segment. On 2026-07-15 the constituent list was re-selected under the same rules after a major sales-history backfill completed the market picture — the list now reflects the cards that would have qualified at launch had the full data been available; the index history was rebuilt from the same inception (2025-03, base 1000).