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Impact factors and Physical Review Letters

by PhilipJ on 10 February 2009

Physical Review Letters has traditionally been one of the top pure physics journals. Sure, Nature and Science are for exciting physics research as well, but there is something dignified and respectable about getting a Letter published in PRL. It isn’t, however, a heavy hitter in the impact factor game. As of 2008 it has a respectable impact factor of around 7, but compared to values of high 20s for Nature and Science, and the mid-to-high teens for the newcomers Nature Physics and Nature Materials, it might seem as if the PRL “brand” has suffered a little.

An editorial titled Is PRL Too Large to Have an ‘‘Impact’’? in this week’s PRL addresses some of the issues surrounding the journal and the impact factor. Not surprising to me, that single number fails to tell the whole story.

In particular, the goal of PRL is to publish “important fundamental research in all areas of physics”, and has been continually expanding its scope to include the disparate fields like biological physics that have really only emerged in the past decade. It now publishes some 4000 Letters each year, despite an increasing rejection rate.

The editorial goes on to show something like an “impact factor density” for the year 2006, or what the impact factor could be if the journal knew a priori just how many citations their articles would get, and limited the number of articles published to only those that would garner these high citations. The data point for Reviews of Modern Physics is thrown in to show what happens when you only publish 30-odd papers a year.

PRL impact factor density

The “long tail” in this distribution is very long indeed: to get the impact factor for PRL up in the teens, they would have to publish approximately a fifth as many articles as they currently do. What’s also interesting, however, is that the top 500 or so PRL articles if published on their own would have an impact factor in the 20s, significantly higher than any other journal which covers just physics.

The editors at PRL had this to say about the data:

[E]ven a journal that publishes very many highly cited papers cannot achieve an impact factor above 20 in physics unless it publishes at most only a few hundred papers per year. The averaging process that the impact factor entails means that attracting even dozens of stellar papers each year will not significantly increase the impact factor of a journal that publishes thousands of papers. Newer bibliometric measures, such as the h-index, the Eigenfactor, etc., provide different perspectives.

And what are they trying to do to improve the journal’s image?

[W]e are not about to ramp up its impact factor in the short term to, say, twice its current value. To do so would entail drastically changing the scope of what we publish. However, as noted above, each year PRL contains several hundred Letters that as a set surpasses in citations any other physics journal in the world. We hope to attract even better submissions and more readers by highlighting this set. The new online publication Physics and PRL Suggestions are significant steps in that direction.

This is an incredibly refreshing take on the impact factor game. Obsessing about a single number from a flawed metric should never change the scope of a journal as esteemed as PRL. I only hope other editors start taking a similar stance.

  1. Alex    3446 days ago    #

    PRL is a place where I can go and read an article on a high precision test of a fundamental theory. I can read a very insightful analysis of an important problem with new and important results. It may not be the hawtest trendiest buzzwordiest nano-bio-metamaterial-blah-blah stuff that will get the most citations from the biggest community out there, but it is very good physics. It is important to keep publishing that stuff.

    One important aspect of impact factor is that the size of the community matters. The bloggers over at DrugMonkey can turn their noses up at impact factors of 6 while biophysicists are happy to get into Biophysical Journal with its 4-5 IF, and optics folks are happy with Optics Letters and Optics Express in the same ballpark. If PRL were to go after the biggest IF they’d have to ditch entire subfields. It’s good that they’re rejecting such a path.

  2. MRW    3445 days ago    #

    I like your main point about not chasing impact factors, but I think that there are a few things you should have mentioned about your analysis:

    1) Reviews of Modern Physics has a high impact factor mostly because it is a review journal. Reviews tend to have higher impact factors than research articles, so a journal specializing in them tends to have a higher impact factor. It’s not clear to me how much the number of articles is involved in this case.

    2) It’s more complicated than just cutting back the number of articles. You have to keep the right articles, which is difficult to do.

    3) The same structure holds for many journals. Science, for example, owes its high impact factor to a relatively small number of extremely highly cited articles.

  3. PhilipJ    3444 days ago    #

    Hi MRW — That’s exactly the point PRL is making, that if they restricted their acceptance to the level of Science or Nature, that they very well could have an impact factor of similar value. Of course, as was said, they’d need to know a priori which Letters to accept.

    I think the interesting thing to note about the RMP number is how small it is given the number of articles published. They state only 30 articles were published that year, yet the IF of RMP was only in the 30s. If you compare this to the data for PRL itself, the IF could be significantly higher. So as often as review articles seem to get cited, novel high impact research gets cited significantly more.

  4. MRW    3443 days ago    #

    I’m not sure that’s true. Nature manages an impact factor of 29 with about 1000 articles published per year.

    Lets assume that the PRL editors and Nature editors can do a good job of predicting what articles will have the most citations.

    It looks to me like you’s have to get to not much more than n = 300 on the graph for PRL to reach an impact factor of 29. Being generous and calling it 400 would mean only 200 articles per year would be published.

    I certainly haven’t done any real analysis, but poking around on ISI, my impression is that Nature’s impact factor would probably be over 60 if only the top 200 articles per year were included.


    As for RMP, I think that what should be learned from it falling well below the curve is that cutting back the number of articles is not going to get you anywhere near the improvement that finding a point on the curve would predict, and that it would probably take cutting down to something far, far less than 200 articles per year for PRL to achieve an impact factor close to Nature.

    Even with publishing only reviews, which are generally higher impact, RMP doesn’t come anywhere close to the curve.

  5. PhilipJ    3443 days ago    #

    That’s not a fair comparison, what you actually want to compare is the impact factor of the physics papers published in Nature, or to Nature Physics. I think the editors were implying a very favourable comparison to Nature Physics in particular.

    When you compare to Nature, it includes all the potentially much higher citations from biology.

  6. ponderingfool    3437 days ago    #

    Late jump in, but sounds like PRL is similar to the Journal of Biological Chemistry with regard to Impact Factors and their response to it. They don’t want to change what the journal is about to play the game to increase the impact factor which I applaud. Both PRL and JBC as one would guess do well when the Page Rank metric is used in place of Impact Factor.

    JBC editorial on IF

    Journal Status

  7. PRLottory    3424 days ago    #

    I had six PRL publications, and recently have dreadful expereinces with PR Lottory. The same papers got rejected by PRL got published elsewhere, with higher IF (three times). And once I almost got it to Nature Physics. Sometimes I am wondering what is wrong with PRL.

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