点击上方“蓝字” 一键订阅近日，Science主编Jeremy Berg在线发表题为“Journal impact factors –Fitting citation distribution curves”的社论，宣告Science集团将推出新的期刊评价方式，改造期刊影响因子。1、期刊影响因子到底是什么，起到什么作用？Journal impact factors are used as metrics for the quality of academic journals. In addition, they are used as metrics for individual publications or individual scientists (see my editorial in Science). The journal impact factor is defined as the average number of times articles published in a given journal over the past 2 years are cited in a given year. This average is derived from a relatively broad distribution of publications with different numbers of citations. Recently, Larivière et al. posted on BioRxiv a proposal recommending sharing these full distributions. This manuscript includes 2015 distributions for 11 journals (in a readily downloadable format). The distribution for Science magazine is shown below:期刊影响因子到底是什么？对于搞科研搞学术的“同志们”来说，对其是再熟悉不过了。大家都知道：期刊影响因子是用来评估、评判学术期刊质量的优劣，衡量一家出版社好坏或是度量一位科学家学术水平高低的重要指标。定义如下：某期刊前两年发表的论文在该报告年份（JCR year）中被引用总次数除以该期刊在这两年内发表的论文总数（即平均应用次数）。但需要注意的是，期刊影响因子的这个“平均数”是包含了来自各式各样的期刊，论文被引用次数的总和所求得的结果。影响因子是否能“真正”反映一篇文章的档次、一位科研工作者的科学成果价值，一直以来都是饱受争议的，婆说婆有理，公说公有理……如果影响因子不是最后最好的计量指标，难道还有其他的、更好的计量方法吗？答案是：有！最近，Larivière等人在生物学预印网站BioRxiv联合发表了一篇文章中说，建议推荐使用一种名叫期刊“引用分布曲线”或叫期刊“频率曲线”来进行评估，并且也可以计算出 “新的”期刊影响因子。这篇联合完成的文章呈现了11种期刊（包括PLoS Biology、PLoS Genetics和PLoS ONE等）在2015年的期刊“引用分布曲线”（网上可供下载）。下面是Science期刊的“引用分布曲线”：Note that the point at 100 represents the sum of the numbers of all papers that received 100 or more citations.从图上我们可以看出，Science的“引用分布曲线”近似服从“左偏态分布”，相比于期刊影响因子，绝大多数文章具有相对较少的引用，只有少数文章获得了比较多的引用次数。需要注意的是，末尾突然上扬的那一点表示：有时（相对来说此情况比较少）也会出现有一批数量很多而且引用次数也很高的文章，这也是我们在期刊影响因子定义下常说的，这段时间出来了很多“爆文”。2、双因子负指数函数拟合期刊“引用分布曲线”This curve rises quickly and then falls more slowly. As a chemist, this reminded me of the curves representing the concentration of an intermediate B in a reaction of the form A -> B -> C.从上图我们已经看到期刊的“引用分布曲线”类似于“左偏态分布”，而不同于指数函数。它是一条先快速上升然后又突然下降的曲线。正如一位化学家所说的那样：“它（“引用分布曲线”）让我想起了在化学上那种逐渐向中间项‘B’聚拢又逐渐消退的‘A -> B -> C’化学反应形式”The concentration of B rises when A is converted to B and then falls when B is transformed into C.Solving equations for the kinetics of this scheme results in a function that is the difference between two exponential functions with negative exponents, that is, P(c) = N*(exp(-k1c) – exp(-k2c)) with k1 < k2.具体来说就是，反应逐渐向中间态‘B’（小纳君称为聚合态）转变，知道初始态‘A’转变为聚合态‘B’，随后又从聚合态‘B’逐渐转变为最终态‘C’。如果用动力学来描述这一过程的话，可以用两个不同系数（双因子）的负指数函数组合而成的数学函数来表示：P(c) = N*(exp(-k1c) – exp(-k2c))且(k1 < k2)。Here, c is the number of citations, P(c) is the population of papers with c citations, k1 and k2 are adjustable constants, and N is a scale factor. The curve rises with an initial slope proportional to (1/k2 – 1/k1) and falls expontially approximately as exp(-k1c).其中，c表示引用的次数，P(c)表示与引用次数相对应的文章数量，k1和k2是调节系数，N是比例因子。曲线起始段上升的斜率正比于系数(1/k2 – 1/k1)，末尾段下降的趋势有点像负指数函数exp(-k1c)。Before fitting the citation curve to this function, we first normalize the curve so that the area under the curve is 1.0 and the y-axis is the fraction of the number of total papers.在对“引用分布曲线”拟合成上式函数以前，首先，需要将曲线进行归一化处理，即把文章数变成总文章数的百分比，防止某一数据过大或过小对分布的影响。This normalized curve can now be fit to the difference of exponential functions. It is easy to show that the normalization constant for the difference of exponential functions is N = k1k2/(k2 – k1) (see mathematical appendix).经过归一化处理后的曲线就可以拟合成两个系数不同的负指数函数的形式。最后得到很容易得到该函数P(c) = N*(exp(-k1c) – exp(-k2c))且(k1 < k2)的归一化系数N = k1k2/(k2 – k1)。（具体推导结果见附件1）The best fit occurs with k1 = 0.05 and k2 = 0.19.最后，拟合出来的调节系数k1和k2分别为0.05和0.19。The apparent journal impact factor can be calculated from these parameters (See mathematical appendix). It can be shown that the journal impact factor (JIF) is: JIF = (k1 + k2) / k1k2.而且，还可以从以上参数中得到期刊影响因子（具体推导结果见附件1）的表达式，即JIF = (k1 + k2) / k1k2。The calculated JIF = 25.3.经过计算求得的期刊影响因子为25.3。Note that this value is smaller that the journal impact factor that is reported (34.7). This is because highly cited papers (with more than 100 citations) have a substantial effect on the journal impact factor but are not well fit by the difference of exponential functions.我们可以发现根据求解期刊“引用分布曲线”的方法所计算出来是期刊影响因子比汤森路透（Thomson Reuters）出品的期刊引证报告（JCR）中所给出的34.7这一数值低很多。这是因为，在传统量化指标计算的方法里，引用高的文章（一般指引用量超过100的文章）也就意味着具有高的影响因子，但是在期刊“引用分布曲线”的方法，由于双因子负指数函数具有2个调节系数k1，k2，因此这就削弱了引用高文章对JIF的贡献量，起到一种平衡高音用量文章的作用，自然算出来是结果也会偏小。3、其它10种期刊的拟合结果With this fitting protocol in place we can now fit the distributions for the other 10 journals.基于以上提出的期刊“引用分布曲线”方法，下面列出了其它10种期刊的拟合“引用分布曲线”结果：1.NatureThe best fit occurs with k1 = 0.07 and k2 = 0.08.调节系数k1和k2分别为0.07和0.08。The calculated JIF = 26.8.期刊影响因子为26.8。2.eLifeThe best fit occurs with k1 = 0.16 and k2 = 0.65.调节系数k1和k2分别为0.16和0.65。The calculated JIF = 7.8.期刊影响因子为7.8。3.PLOS ONEThe best fit occurs with k1 = 0.31 and k2 = 2.调节系数k1和k2分别为0.31和2。The calculated JIF = 3.7.期刊影响因子为3.7。4.PLOS BiologyThe best fit occurs with k1 = 0.16 and k2 = 0.57.调节系数k1和k2分别为0.16和0.57。The calculated JIF = 8.0.期刊影响因子为8。5.PLOS GeneticsThe best fit occurs with k1 = 0.18 and k2 = 0.92.调节系数k1和k2分别为0.18和0.92。The calculated JIF = 6.6.期刊影响因子为6.6。6.Nature CommunicationsThe best fit occurs with k1 = 0.13 and k2 = 0.66.调节系数k1和k2分别为0.13和0.66。The calculated JIF = 9.2.期刊影响因子为9.2。7.EMBO JournalThe best fit occurs with k1 = 0.16 and k2 = 0.37.调节系数k1和k2分别为0.16和0.37。The calculated JIF = 9.0.期刊影响因子为9。8.Proceedings of the Royal Society of London BThe best fit occurs with k1 = 0.24 and k2 = 1.42.调节系数k1和k2分别为0.24和1.42。The calculated JIF = 4.9.期刊影响因子为4.9。9.Journal of InformetricsThe best fit occurs with k1 = 0.32 and k2 = 2.调节系数k1和k2分别为0.32和2。The calculated JIF = 3.6.期刊影响因子为3.6。10.Scientific ReportsThe best fit occurs with k1 = 0.22 and k2 = 2.调节系数k1和k2分别为0.22和2。The calculated JIF = 5.0.期刊影响因子为5。4、计算结果分析The calculated journal impact factors are well correlated with the observed values as shown below:由本文提出的期刊“引用分布曲线”方法求解出的期刊影响因子与所给出的期刊影响因子的大致趋势是保持一致的，二者的分布结果如下图所示：A line with slope 1 is shown for comparison. The overall Pearson correlation coefficient is 0.999. Fitting all 11 data points to a line through the origin yields a slope of 0.746. The fact that this slope is substantially less than 1 is largely driven by the values for Science and Nature which, as noted above, are lower than the reported values owing to the elimination of the effect of papers with more than 100 citations. If these two points are eliminated, the slope of a fitted line increases to 0.924.为了便于比较，图中的画了一条斜率为1的直线，汤森路透（Thomson Reuters）所给出的期刊影响因子分布与斜率为1的直线之间的相关系数为0.99，即汤森路透（Thomson Reuters）所给出的期刊影响因子分布的斜率为0.99。而用期刊“引用分布曲线”方法所计算出来的11家期刊影响因子（包含Science and Nature的情况）分布也大致呈一条直线，且斜率为0.746。这条直线的斜率与1相差很多，这是由于Science和Nature这两家期刊用文中提出的方法所计算出来影响因子远远小于汤森路透（Thomson Reuters）JCR报告中所给出的数值所造成。若在不包含Science and Nature两家期刊的情况，直线的斜率为0.924，这个结果已经和汤森路透（Thomson Reuters）所给出的结果相当接近了。5、总结We have demonstrated that a function formed as the difference of two exponential functions can be used to fit observed distributions of the numbers of papers with different number of citations. Fitting this functional form to data from 11 journals reproduces the curves well and generates journal impact factors that agree well with published values. The largest differences are in journals such as Science and Nature that have substantial numbers of papers with more than 100 citations over the 2-year period. This emphasizes again how these outlier papers can affect journal impact factor values.我们已经提出了用双因子的负指数函数来拟合文章数与引用次数之间的分布关系。从11加期刊的拟合结果来看，期刊“引用分布曲线”取得了较好的拟合效果，而且用该方法求得的期刊影响因子与汤森路透（Thomson Reuters）所给出的期刊影响因子大致保持一致，只有Science和Nature两家期刊求得的结果偏小。像Science和Nature这样的期刊，在2年内会出现有一大批数量很多而且引用次数超过100的“爆文”。正是这些“奇异”的爆发点起到了推波助澜的作用，从而抬高了本没有“那么高”期刊影响因子。6、附录：公式推导期刊影响因子可以从文章数与引用次数之间的期刊“引用分布图”推导出来。期刊“引用分布图”可以进行归一化，即把文章数变成总文章数的百分比与引用次数之间的“引用分布图”。归一化后的曲线可以用以下函数形式表示：其中，c为引用次数，P为一定引用数下所对用的总文章数的百分比，N为归一化常数，而且英文原文： 来源：百纳知识 免责声明：部分资料来源于网络，转载的目的在于传递更多信息及分享，并不意味着赞同其观点或证实其真实性，也不构成其他建议。仅提供交流平台，不为其版权负责。如涉及侵权，请联系我们及时修改或删除。邮箱：info@polymer.cn诚邀投稿欢迎专家学者提供稿件（论文、项目介绍、新技术、学术交流、单位新闻、参会信息、招聘招生等）至info@polymer.cn，并请注明详细联系信息。高分子科技®会及时推送，并同时发布在中国聚合物网上。欢迎加入微信群 为满足高分子产学研各界同仁的要求，陆续开通了包括高分子专家学者群在内的几十个专项交流群，也包括高分子产业技术、企业家、博士、研究生、媒体期刊会展协会等群，全覆盖高分子产业或领域。目前汇聚了国内外高校科研院所及企业研发中心的上万名顶尖的专家学者、技术人员及企业家。申请入群，请先加审核微信号chemshow（或长按下方二维码），并请一定注明：高分子+姓名+单位+职称（或学位）+领域（或行业），否则不予受理，资格经过审核后入相关专业群。点击下方“阅读原文”查看更多