影响因子= impact factor = (前2年发表在本杂志上的文章在今年被引用的次数)除以(前2年发表在本杂志上的文章数目)

比如A杂志在2007年的影响因子是这么计算的: 2005和2006年A杂志上总共发表了500篇文章, 2005和2006年发表的这500篇文章在2007年被引用了1345次, 那么A杂志的2007年影响因子是1345/500=2.69.

国内很多地方以影响因子为唯一依据其实是有很多不足的地方. 至少有2个不足: (1) 首先各个专业不一样, 每个专业的好杂志是不一样的,因为小专业的杂志当然被引用的次数就没有大专业多,大专业写文章的人也多啊. (2) 临床和基础的不一样, 临床的很多文章是解决诊断和治疗问题,所以很多人用但是被人写文章引用的机会就小些. 

国情

原来如些

A journal can adopt editorial policies that increase its impact factor.^[12] These editorial policies may not solely involve improving the quality of published scientific work.

• Journals may publish a larger percentage of review articles which generally are cited more than research reports.^{[citation needed]} Therefore review articles can raise the impact factor of the journal and review journals will therefore often have the highest impact factors in their respective fields.
• Journals may change the fraction of "citable items" compared to front-matter in the denominator of the IF equation. Which types of articles are considered "citable" is largely a matter of negotiation between journals and Thomson Scientific. As a result of such negotiations, impact factor variations of more than 300% have been observed.^[13] For instance, editorials in a journal are not considered to be citable items and therefore do not enter into the denominator of the impact factor. However, citations to such items will still enter into the numerator, thereby inflating the impact factor. In addition, if such items cite other articles (often even from the same journal), those citations will be counted and will increase the citation count for the cited journal. This effect is hard to evaluate, for the distinction between editorial comment and short original articles is not always obvious. "Letters to the editor" might refer to either class.
• Several methods, not necessarily with nefarious intent, exist for a journal to cite articles in the same journal which will increase the journal's impact factor.^[14]
• In 2007 a specialist journal with an impact factor of 0.66 published an editorial that cited all its articles from 2005 to 2006 in a protest against the absurd use of the impact factor.^[15] The large number of citations meant that the impact factor for that journal increased to 1.44. As a result of the increase, the journal was not included in the 2008 Journal Citation Report.^[16]

• The impact factor is often misused to evaluate the importance of an individual publication or evaluate an individual researcher.^[17] This does not work well since a small number of publications are cited much more than the majority - for example, about 90% of Nature's 2004 impact factor was based on only a quarter of its publications, and thus the importance of any one publication will be different from, and in most cases less than, the overall number.^[18] The impact factor, however, averages over all articles and thus underestimates the citations of the most cited articles while exaggerating the number of citations of the majority of articles. Consequently, the Higher Education Funding Council for England was urged by the House of Commons Science and Technology Select Committee to remind Research Assessment Exercise panels that they are obliged to assess the quality of the content of individual articles, not the reputation of the journal in which they are published.^[19]

以下是引用月新在2008-12-7 22:46:00的发言：   中国许多小杂志几乎全靠自引，自引就是自己引用自己的文章，如果不自引，别人根本不会引用，因此自引率算上，也有一定的影响因子。我前三年发表一篇文章，杂志社逼我必需引他们杂志一篇文章，我当时不明白，非常气愤，现在才明白，理解他们的苦心。这就是自引。非常可怕的做法。 等于自己扯自己的头发上天。

把自引的次数从引用次数中去掉后计算出来的叫调整后的影响因子adjusted impact factor.

自引的现象在很多杂志都有, 所有专业的杂志都包括的话平均自引率是12%左右, 高的有些达到25%.