Introduction

“Speed” has been highlighted as one of the most important characteristics of altmetrics (Wouters and Costas 2012; Bornmann 2014). Compared to citations, which has been often criticized for its time delay in providing reliable measurement for research impact (Wang 2013), speed in the context of altmetrics is related to the idea that the impact of a given scientific output can be measured and analyzed much earlier (Priem et al. 2010; Mohammadi and Thelwall 2014). Publication delays are considered to substantially slow down the formal communication and dissemination of scientific knowledge (Amat 2008; Björk and Solomon 2013). In contrast, scholarly interactions on social media platforms are likely to happen within a very short time-frame. For instance, Twitter mentions of scientific documents may occur immediately within hours or even minutes after they were available online (Shuai et al. 2012; Haustein et al. 2015a).

However, because of the strong heterogeneity of altmetrics (Haustein 2016), which incorporate a wide range of metrics based on different types of data sources, it is difficult to establish a clear-cut and unified conceptual framework for the temporal analysis of all altmetrics. Each altmetric indicator, typically with unique functions and aimed at different audiences, may tell different stories about the reception of publications, and show distinguishing patterns in varying contexts. Lin and Fenner (2013) concluded that altmetrics are very likely representing very different things. From this point of view, we argue that the interpretation of the characteristic properties of different altmetrics should be made for each metric separately, including among these properties also their “speed”.

Accumulation patterns and immediacy measurement of citations and usage metrics

In contrast to altmetric data, the accumulation patterns of citations have already been widely discussed in previous studies from several perspectives, such as their “obsolescence” (Line 1993), “ageing” (Aversa 1985; Glänzel and Schoepflin 1995), “durability” (Costas et al. 2010), or “delayed recognition” (Garfield 1980; Min et al. 2016). Citation histories, which relate to the analysis of the distribution of citations over time, were mainly studied from the synchronous or diachronous perspectives (Stinson and Lancaster 1987). The former considers the distribution of the publication years of cited references, while the latter focuses on the distribution of received citations over time (Colavizza and Franceschet 2016; Sun et al. 2016), which are also referred to as “retrospective citations” and “prospective citations”, respectively (Glänzel 2004). These two approaches have been applied to studying the accumulation patterns of usage metric data as well. With the development of digital publishing, usage metrics have been proposed and adopted by publishers during the last decades to supplement citations in reflecting how frequently scientific outputs are used and measuring their early impact to some extent (Schloegl and Gorraiz 2011). From the synchronous perspective, Kurtz et al. (2005) concluded that most studies of obsolescence found that the use of literature declines exponentially with age. The diachronous accumulation patterns of usage metrics, like views, downloads, reads, etc., were investigated and often compared with citations. On the basis of page views data of Nature publications, Wang et al. (2014) explored the dynamic usage history over time and found that papers are used most frequently within a short period after publication, finding that in median it only takes 7 days for papers to reach half of their total page views. Schlögl et al. (2014) reported that citations take several years until they reach their peak, however most downloads of papers are quickly accrued in the same publication year. In a similar fashion, Moed (2005) already found that citations and downloads show different patterns of obsolescence, and about 40% of downloads accumulated within the first 6 months after publication. More recently Wang et al. (2016a) using the article-level “usage counts” provided by Web of Science to investigate the usage patterns of indexed papers, identified that newly published papers accumulated more Web of Science usage counts than older papers.

As to the measurement of the “speed” of citations and usage metrics, several indicators have been created and applied in practice. For example, based on the time elapsed between the publication date and the date of the first citation of a paper, Schubert and Glänzel (1986) developed the indicator mean response time (MRT) in order to measure the citation speed of journals, understood as the properly formed average number of years between the publication of articles in a journal and the time of their first citation. In order to measure how quickly articles in a journal are cited, the Journal Citation Reports (JCR) calculates the indicator named Immediacy Index for each journal in each year. This indicator is defined as the average number of times an article is cited in the same year it is published.Footnote 1 Besides, at the journal level, Cited Half-Life and Citing Half-Life are also calculated by JCR to measure how fast journals are accumulating half of their citations and how far back that citing relationship extends.Footnote 2 Analogous to the citation-based Immediacy Index and half-life, the “usage immediacy index” and “usage half-life” (Rowlands and Nicholas 2007), “download immediacy index” (Wan et al. 2010) were proposed to describe the life cycle of usage metrics. By analyzing usage data in the field of oncology collected from Science Direct, Schloegl and Gorraiz (2010) calculated the mean usage half-life and found that it is much shorter than the average cited half-life, observing also different obsolescence patterns between downloads and citations.

Accumulation patterns and immediacy measurement of altmetric data

Since the emergence of altmetrics, most related studies have focused on the coverage of publications across altmetric sources and their correlation with citation counts (Thelwall et al. 2013; Haustein et al. 2014; Costas et al. 2015a). Less attention was paid to the study of the accumulation velocity of altmetric data over time. Only a few altmetric data sources were investigated from the perspective of their immediacy. Maflahi and Thelwall (2018) conducted a longitudinal weekly study of the Mendeley readers of articles in six library and information science journals and found that they start to accrue early from when articles are first available online and continue to steadily build over time, being this the case even for journals with large publication delays. Thelwall (2017) also found that articles attracted between 0.1 and 0.8 Mendeley readers on average in the month they first appeared in Scopus, with some variability across subject fields. The results based on PeerJ social referrals data of Wang et al. (2016b) suggested that the number of “visits” to papers from social media (Twitter and Facebook) accumulates very quickly after publication. By comparing the temporal patterns of Twitter mentions and downloads of ar** altmetric events separate seems to be an important recommendation, this given not only their fundamental differences (Haustein et al. 2016; Wouters et al. 2019) but also their time accumulation patterns as demonstrated in this study. Moreover, the pace and tempo of different altmetrics cannot be seen as equivalent and, similar to what happens with citations, these time differences need to be taken into account when considering different time windows in altmetric research.

Variations across document types

Zahedi et al. (2014) concluded that the coverage of several altmetric data sources varies across document types and subject fields. In this study, it is shown that the same type of variations apply also to the data accumulation velocity of different altmetric data sources. Thus, in terms of document types, Reviews (this document type mainly focuses on retrospectively reviewing existing findings) are overall the slowest in accumulating altmetric events. A possible reason for this slowest reception lies in the less innovative nature of Reviews. In other words, Review papers are less prone to provide new research discoveries and more to condense the state-of-the-art in a subject field or research topic, therefore lacking the novelty component of other document types. For example, the research topics presented in Editorial Materials and Letters may be more likely to evoke social buzz immediately, since they cover more novel topics, debates, scientific news, etc., without using a too complicated and technical language (Haustein et al. 2015). The thematic property of these two document types might facilitate the users’ attention received more immediately, particularly on Peer review platforms, a type of altmetric data source which is mainly used by researchers, who are faster to take notice of controversial topics emerging in the scientific community. This finding is quite similar with the ageing patterns of citations to different document types: Editorial Materials and Letters were found more likely to be the “early rise-rapid decline” papers with most citations accumulated in a relatively short time period, while Review was observed to be the delayed document type with a slower growth (Costas et al. 2010; Wang 2013).

Variations across scientific fields and topics

In terms of scientific fields, research outputs from the fields of Physical Sciences and Engineering and Life and Earth Sciences are more attractive to social media audiences shortly after publication, accruing altmetric events faster compared to other fields. Research outputs from the fields of both Social Sciences and Humanities and Mathematics and Computer Science are relatively slower to be disseminated on altmetric data sources, although publications in these two fields hold different altmetric data coverage, with the former much higher than the latter (Costas et al. 2015a). Such field-related data accumulation dynamics was also observed in the context of citations, for instance, citation ageing in the social sciences and mathematics journals is similarly slower than in the medical and chemistry journals (Glänzel and Schoepflin 1995), the physical, chemical, and earth sciences, fields in which the research fronts are fast-moving, have more papers showing rapidly declining citation pattern (Aksnes 2003). From the perspective of first-citation speed, papers in the field of physics are faster in receiving the first citation, followed by biological, biomedical, and chemical research, while mathematics papers show lower first-citation speed (Abramo et al. 2011). Even though the overall accumulation patterns between citation data and most altmetric data are obviously different, they share very similar tempos across scientific fields.

Furthermore, the variations do not only exist at the main subject field level, but also the research topic level. Within each subject field, different research topics also show various velocity patterns in receiving altmetric attention, both on fast sources or slow sources. This signifies the thematic dependency of users in following up-to-date research outputs around some topics, just like some certain research topics drive more social attention over others (Robinson-Garcia et al. 2019). Thus, further research should focus on identifying the main distinctive patterns of publications and research topics to determine their faster/slower reception across altmetric sources, and how different observation time windows, and the selection of different data sources, may affect real-time assessment in altmetric practice.

Limitations

The main limitation of this study lies in the precision of Crossref’s DOI created date as the proxy of actual publication date of research outputs. There might still be a small distance between the date on which a DOI was created and the research output was actually made publicly available, which could result in some inaccuracies in our results. Besides, as we mentioned in the data part, DOI created dates might be updated due to the change of DOI status, thereby causing the unreliable time intervals. One of the effects of these inaccuracies is that some publications may have altmetric event posted date even earlier than DOI created dates. Therefore, publications with such unexpected time intervals have been excluded from this study to lower the negative influence made by questionable DOI created dates. Future research should focus on refining accurate methods of identifying the effective publication date of research outputs. As shown in this study, they have important repercussion to determine accurate time windows for altmetric research.

Conclusions

Several conclusions can be derived from this study. First, we conclude that not all altmetrics are fast and that they do not accumulate at the same speed, existing a fundamental differentiation between fast sources (e.g. Reddit, Twitter, News, Facebook, Google+, and Blogs) and slow sources (e.g. Policy documents, Q&A, Peer review, Wikipedia, Video, and F1000Prime). Another important conclusion of this study is that the accumulation velocity of different kinds of altmetric data varies across document types, subject fields, and research topics. The velocity of most altmetric data of Review papers is lower than that of Articles, while Editorial Material and Letter are generally the fastest document types in terms of altmetric reception. From the perspective of scientific fields, the velocity ranking of different data sources changes across subject fields, and most altmetric data sources show higher velocity values in the fields of Physical Sciences and Engineering and Life and Earth Sciences, and lower in Social Sciences and Humanities and Mathematics and Computer Science. Finally, with regards to individual research topics, substantial differences in the velocity of reception of altmetric events across topics have been identified, even among topics within the same broader field. Such topical difference in velocity suggests that it is worth studying the underlying reasons (e.g. hotness, controversies, scientific debates, media coverage, etc.) of why some topics within the same research area do receive social (media) attention much faster than others.