Methodology
How the index is built, how journals are classified by subject, and how the POSI Citation Score and the journal rankings are calculated.
1. Sources and update cycle
The journal index is built from two registries. OpenAlex describes about 207,000 journals, with their topics, output and citation history. Crossref lists about 171,000 journals that register DOIs. The two are merged on ISSN: a journal appears once, under its linking ISSN (ISSN-L), whichever registry describes it. Journals without an ISSN are left out, because without one a journal cannot be identified reliably.
Journal metadata is refreshed every day. Rankings are recomputed in cycles: each cycle harvests both registries, computes PCS for every indexed journal and publishes a new ranking edition. Publication search queries OpenAlex directly, so publication records are always current.
2. Subject classification
Each journal is assigned one category of the POSI Subject Classification (PSC): 42 categories in six domains. The assignment is computed from the journal’s OpenAlex topics. Each topic is mapped to a PSC category through a fixed crosswalk, and topic counts are summed per category.
| Assignment | Rule | Ranked by subject |
|---|---|---|
| High confidence | The leading category holds at least 35% of the journal’s topic mass, at least 1.5 times the next category, and the journal has at least 50 works. | Yes |
| Provisional | A category leads, but on fewer than 50 works, or without a clear margin over the next category. | No |
| Multidisciplinary | No category and no domain dominates, as with Science, Nature or The Lancet. Listed under Multidisciplinary. | No |
| Not classified | No topic data, usually a journal registered with Crossref only. | No |
A subject editor may confirm a classification. A confirmed classification counts as high confidence. Confirmation records whether the classification is correct; it never moves a journal to a category where it would rank better.
3. POSI Citation Score (PCS)
PCS is the mean number of citations received by a journal’s citable items published in the four complete years before the metric year. For the 2026 edition, that is items published from 2022 to 2025.
PCS = total citations to eligible items ÷ number of eligible items
- Citations are Crossref’s citation count for each DOI.
- Eligible items are research articles, reviews and other citable document types. Editorials, corrections and similar items are excluded.
- Every eligible item in the window is counted. There is no sampling.
- A work with no citation data is counted separately from a work with zero citations, and reported in the journal’s coverage figure.
Crossref’s citation data depends on what publishers deposit, so coverage varies between publishers and fields. PCS is a Crossref-observed citation indicator, not a census of every citation.
4. Ranks, percentiles and quartiles
Journals are ranked by PCS in two ways: within their subject category, and across all journals. The ranking method is the same in both.
- Journals are ordered by PCS, highest first.
- Journals with the same PCS share the mid-point of the positions they occupy.
- The percentile is
100 × (N − rank + 0.5) ÷ N, where N is the number of ranked journals. - The quartile follows from the percentile: PCS-Q1 at 75 or above, PCS-Q2 at 50 or above, PCS-Q3 at 25 or above, otherwise PCS-Q4.
Quartiles are always labelled PCS-Q1 to PCS-Q4, so they cannot be confused with quartiles published by other services.
5. Ranking eligibility
Core Collection and indexed journals are ranked under the same rules. A journal receives an overall rank when:
- PCS has been computed for the current window;
- it has at least 5 eligible items in the window; and
- citation data was retrieved for at least 90% of those items.
A category rank additionally requires:
- a high-confidence or confirmed subject classification (multidisciplinary journals are ranked overall only); and
- at least 20 ranked journals in the category.
A journal that is not ranked shows the first rule it did not meet on its profile page.
6. Versions
Every published value carries the methodology version that produced it. The current versions are ranking PCS-Q-1.0, citation score PCS-1.0 and subject crosswalk PSC-CROSSWALK-0.3. A change to any formula or threshold is published as a new version and announced in news; earlier editions are not recalculated silently.
For how records are verified and sourced, see provenance and verification.