PubMed Central’s role in readership amplification

Many of Hindawi's leading journals are indexed in PubMed Central - Hindawi Blog

Since PubMed Central (PMC) launched in Feb 2000 it has continued to grow rapidly. This was initially driven through the NIH Public Access Policy which pushed for voluntary deposit of all research funded by the National Institutes of Health (NIH). This voluntary policy was followed by the 2007 Consolidated Appropriations Act of 2008 (H.R. 2764) which included a provision requiring complete electronic copies of NIH-funded peer-reviewed research to be deposited into PubMed Central within 12 months of publication. This policy shift drove a major expansion of the repository, which now contains 4.1 million articles, and made it a core resource for biomedical and life science journal articles

PMC contents

Another outcome of this policy change was publishers working collaboratively with the NIH to make deposits directly into PubMed Central on behalf of their authors. This collaboration ensures that researchers are fully compliant with the requirements of their NIH grant funding without any extra work on their part.

Hindawi is one of the publishers actively collaborating with the NIH, with 187 journals depositing content directly into PubMed Central. As we have over 83,000 articles now available, we were interested in discovering what effect PubMed Central has had on the distribution and use of articles that are already made openly available through publication on

Figure 1 Number of Hindawi articles deposited in PMC

Number of Hindawi articles in PMC

You can clearly see the effect of the 2007 Consolidated Appropriations Act on the deposit of articles into PubMed Central and the rapid growth since that point. This growth in available content has driven an increased level of activity on the PubMed Central site, with average monthly downloads running around 1.5 to 1.7m.

Figure 2 Average monthly downloads of Hindawi Articles on PMC

Downloads on PMC

In that time we have also seen a huge increase in the number of unique users of our content on PubMed Central (as shown in Figure 3). This reflects the growing amount of available content, as well as the increasing popularity of PubMed Central as a full text repository.

Figure 3 Unique users of Hindawi content on PMC

Unique users - PMC

Since 2007, the number of unique users accessing Hindawi content on PMC has grown from less than 50,000 to over 700,000 per month.

Whilst the growth in downloads and unique users on the PubMed Central platform is impressive, it does raise the question of whether this represents actual growth in usage or a movement of usage from one source to another. To investigate this further we analyzed the relative performance of the PMC deposited articles across the two platform; PubMed Central and

Figure 4 Article downloads of journal content deposited in PubMed Central

Comparison vs Hindawi.comFor articles that exist both on and in PubMed Central there is a significant boost to overall usage. PubMed Central is adding 1.5 to 1.7m downloads per month on average, which boosts the overall readership of our articles by approximately 37% (as compared to usage alone).

Finally to check that usage is not just transitioning from one platform to another, we looked at the comparative download performance on of the articles in PMC vs those articles that were not deposited in PubMed Central.

Figure 5 Average article downloads on

comparative article performance

Figure 5 shows the comparison between average downloads per article for PMC articles, non-PMC articles and all articles on The PMC articles far outperform the non-PMC articles and also outperform the average across all articles. Some of this performance difference could be explained by articles disciplines with Biomedical articles (as found in PMC) generally attracting more downloads than other fields. However, with PubMed sending over 100,000 referrals a month to Hindawi (as Illustrated in Figure 6) then it is also likely that this traffic from PubMed is contributing to higher usage levels on, even with the free availability of this content on PubMed Central.

Figure 6 Average monthly referrals from PubMed to

Referrals from PubMed to


From the data that we analyzed, there are a few conclusions that we can draw for Hindawi’s articles deposited in PubMed Central:

  1. Depositing articles in PubMed Central adds around 37% extra downloads compared to alone
  2. There is no evidence of a loss of downloads from the platform for content available in PubMed Central
  3. There is some evidence to show that availability of articles in PubMed Central has increased average downloads of those articles on

We will continue to analyze the performance of our content on PubMed Central but on the basis of our current analysis, full text availability on PubMed Central is boosting the usage of our articles and serving our authors in the best possible way.

The text and images in this blog post are by Hindawi and are distributed under the Creative Commons Attribution License (CC-BY).