Last month we started looking at whether it’s possible to find definitive information about where to publish, and about which journals are fully open access. We looked at the sizes of various indexes to see how comprehensive they are, but noted that size is only part of the story. This month we look at how indexes overlap, and whether we can get to a definitive source.

Background

Some indexes, of course, are more exclusive than others. Different indexes take different approaches and have different objectives. For example, the selection processes used by Cabells, Clarivate, the DOAJ, and Elsevier illustrate the breadth of issues covered when determining journal quality. Even where similar types of criteria are used, their specific application or thresholds may differ.

By analyzing titles that sit in multiple indexes we can determine the overlap and gaps between each index and get a sense of whether we need to shop around across indexes to gather a comprehensive list of titles.

In theory, titles should have ISSNs that uniquely identify them. In practice, one title may have several ISSNs, for example for a print version and an online version. To facilitate working with journal lists, theInternational ISSN Centre (which oversees ISSNs) designates just one ISSN from each journal as its “definitive” identifier, which is known as its linking ISSN, or ISSN-L.

Bielefeld University’s work on producing a definitive list of fully OA ISSNs from public sources uses the ISSN-L to identify and count titles. We have used a similar approach. We linked lists of ISSNs in indexes back to their parent ISSN-Ls, removed duplicates, and then considered a title to sit in two indexes if its ISSN-Ls appeared in each. Despite the 2 million or so ISSN-Ls registered, ambiguities still apply, such as titles with more than one parent ISSN-L, titles with no parent ISSN-L at all, and some with no ISSNs at all.

Relative Index Coverage

We start by looking at the overlaps between indexes for all journals, shown in Table 1, below.

Sources: Indexes as stated, readings taken Jun/Jul 2019. Delta Think analysis. © Delta Think 2019, All rights reserved.

The table lists various indexes down its left-hand side. Reading across the table, it shows what proportion of each index’s titles appear in other indexes. So, for example, 28% of the DOAJ’s titles also appear in Elsevier’s CiteScore. Percentages do not add up to 100% as titles may appear in more than one index.

The n column shows the total number of titles (technically, unique ISSN-Ls) within each index, so you can get a sense of scale. The table also is overlaid with a heatmap.

• The more exclusive JCR shows a significant overlap with the Scopus-derived indexes, suggesting the one is a subset of the other.
• The Price Lists row (which shows Delta Think’s sample of journal APCs from the 30 or so largest publishers), shows that 81% of their journals appear in Scopus and 57% in the JCR.
• The Bielefeld row (showing the Gold OA list mentioned above) is designed to be inclusive, so it has many titles that do not appear in other (more exclusive) indexes.
• PMC is a relatively small index, so only accounts for a small proportion of many indexes. However, the journals in PMC are widely indexed elsewhere.

Fully OA Journals

We can also look at the overlaps for just fully OA journals, as shown in Table 2, below.

Sources: Indexes as stated, readings taken Jun/Jul 2019. Delta Think analysis. © Delta Think 2019, All rights reserved.

The table filters the “Titles in” column for fully OA journals, as designated by their parent indexes. We did not have detailed information for all indexes, so their rows are not shown.

• The dedicated full OA indexes (Bielefeld and DOAJ) show no change in their overlaps, as filtering for OA will not change their lists.
• Most of the JCR’s OA journals are in the DOAJ, compared with 71% of the Scopus-derived ones. This suggest the latter are casting a wider net in some respects than the DOAJ.
• The reverse is true too, with only small proportions of the DOAJ’s journals overlapping with the major indexes.
• The small overlaps between the dedicated fully OA indexes suggest that they are capturing different sets of journals.
• We have excluded CrossRef as a source, as we wish to focus on curated lists. At the time of writing we did not have information about Web of Science ISSNs, however studies have shown a high degree of overlap between WoS and Scopus².

Definitive Coverage

So, is there a one-stop shop? The spread of index coverage is further illustrated in Figure 1, below.

All journals, n=43,840; Fully OA journals, n=15,687.
Source: Delta Think analysis. © Delta Think 2019, All rights reserved.

The figure shows how many different indexes ISSN-Ls appear in, expressed as a proportion of our overall sample. For example (as illustrated in the two left-most grey bars), just over 50% of all ISSN-Ls we analyzed only appear in one index, and around 15% only appear in two indexes. The chart does not analyze whether these are in the same index – it simply analyzes the likelihood of having to look in more than one index to find a given title. The chart does not include the inclusive Bielefeld Gold list, as we are trying to analyze the more selective indexes.

Taken in conjunction with the table showing overlap, the data suggest that no one index can offer anything approaching comprehensive coverage. At least half of all titles, and two-thirds of fully OA ones exist exclusively in only one index, but there is no way to predict which one.

Conclusions

We set out to examine whether there is a definitive, curated list of journals that researchers can use when deciding on their publication venue. While some offer very good coverage, the short answer appears to be that no one index offers a definitive list.

Across all journals, there seems to be overlap of significant proportions of the mainstream indexes. However, fully OA journals present a more varied landscape. You need to combine multiple lists to round up a comprehensive list of curated fully OA journals.

Our analysis has combined over 100,000 ISSNs across over 65,000 titles and, we think it represents one of the most comprehensive round ups of the coverage of curated lists available.

Notes:

1 – When measuring exclusivity, note that CiteScore and SCImago both derive from Scopus data, so we count them as one index to avoid double-counting.

2 – Web of Science coverage overlapping Scopus, see, for example, Mongeon, P. & Paul-Hus, A. Scientometrics (2016) 106: 213. https://doi.org/10.1007/s11192-015-1765-5

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