We’re hosting the international useR! conference at Warwick this summer, and I thought it might be interesting to try to get some data on how the use of R is growing. I decided to look at scholarly citations to R, mainly because I know where to find the relevant information.

I have access to the ISI Web of Knowledge, as well as to Google Scholar. The data below comes from the ISI Web of Knowledge database, which counts (mainly?) citations found in academic journals.

Background: How R is cited

Since version 0.90.0 of R, which was released in November 1999, the distributed software has included a FAQ document containing (among many other things) information on how to cite R. Initially (in 1999) the instruction given in the FAQ was to cite

• Ihaka, R and Gentleman, R (1996). R: A Language for Data Analysis and Graphics, Journal of Computational and Graphical Statistics 5, 299-314

When R version 1.8.1 was released in November 2003 the advice on citing R changed: people using R in published work were asked to cite

• R Development Core Team (2003). R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing, Vienna, Austria.

The “2003” part of the citation advice has changed with each passing year; for example when R 1.9.1 was released (in June 2004) it was updated to “2004”.

ISI Web of Knowledge: Getting the data

Finding the citation counts by searching the ISI database directly does not work, because:

1. the ISI database does not index Journal of Computational and Graphical Statistics as far back as 1996; and
2. the “R Core Development Team” citations are (rightly) not counted as citations to journal articles, so they also are not directly indexed.

So here is what I did: I looked up published papers in the ISI index which I knew would cite R correctly. [This was easy; for example my friend Achim Zeileis has published many papers of this kind, so a lot of the results were delivered through a search for his name as an author.] For each such paper, the citation of interest would appear in its references. I then asked the Web of Knowledge search engine for all other papers which cited the same source, with the resulting counts tabulated by year of publication.

It seems that the ISI database aims to associate a unique identifier with each cited item, including items that are not themselves indexed as journal articles in the database. This is what made the approach described above possible.

There’s a hitch, though! It seems that, for some cited items, more than one identifier gets used. Thus it is hard to be sure that the counts below include all of the citations to R: indeed, as I mention further below, I am pretty sure that my search will have missed some citations to R, where the identifier assigned by ISI was not their “normal” one. (This probably seems a bit cryptic, but should become clearer from the table below.)

Citation counts

As extracted from the ISI Web of Knowledge on 25 June 2011:

ISI identifier ↓	1998	1999	2000	2001	2002	2003	2004	2005	2006	2007	2008	2009	2010	Total
IHAKA R J COMPUTATIONAL GRAP 5 : 299 1996	5	15	18	43	131	290	472	528	435	419	449	378	396	3579
*R DEV COR TEAM R LANG ENV STAT COMP : 2003							39	123	91	57	39	25	14	388
*R DEV COR TEAM R LANG ENV STAT COMP : 2004							16	235	421	327	289	187	126	1601
*R DEV COR TEAM R LANG ENV STAT COMP : 2005								42	397	531	511	445	366	2292
*R DEV COR TEAM LANG ENV STAT COMP : 2005								5	39	75	41	25	10	195
*R DEV COR TEAM R LANG ENV STAT COMP : 2006									55	438	849	656	461	2459
*R DEV COR TEAM R LANG ENV STAT COMP : 2007										92	714	962	733	2501
*R DEV COR TEAM R LANG ENV STAT COMP : 2008											208	1402	1906	3516
*R DEV COR TEAM LANG ENV STAT COMP : 2008											7	21	44	72
*R DEV COR TEAM R LANG ENV STAT COMP : 2009												172	1363	1535
*R DEV COR TEAM R LANG ENV STAT COMP : 2010													205	205
*R DEV COR TEAM R LANG ENV STAT COMP :							1	12	14	25	36	81	93	262
Total	5	15	18	43	131	290	528	945	1452	1964	3143	4354	5717	18605

For the “R Development Core Team (year)” citations, the peak appears about 2 years after the year concerned. This presumably reflects journal review and backlog times.

There are almost certainly some ISI identifiers missing from the above table (and, as a result, almost certainly some citations not yet counted by me). For example, the number of citations found above to R Development Core Team (2009) is lower than might be expected given the general rate of growth that is evident in the table: there is probably at least one other identifier by which such citations are labelled in the ISI database (I just haven’t found it/them yet!). If anyone reading this can help with finding the “missing” identifiers and associated citation counts, I would be grateful.

The graph shows the citations found within each year since 1998.

[Click on the graph to view it at a larger size.]

Citations to Ihaka and Gentleman (1996) and to R Core Development Team (any year) are distinguished in the graph, and the total count of the two kinds of citation is also shown.