The Institute for Scientific Information (ISI) impact factor has become an important standard for assessing journal quality. Here we propose that impact factors may be subject to inflation analogous to changes in monetary prices in economics. The possibility of inflation came to light as a result of the observation that papers published today tend to cite more papers than those published a decade ago. We analyzed citation data from 75,312 papers from 70 ecological journals published during 1998–2007. We found that papers published in 2007 cited an average of seven more papers than those published a decade earlier. This increase accounts for about 80% of the observed impact factor inflation rate of 0.23. In examining the 70 journals we found that nearly 50% showed increases in their impact factors, but at rates lower than the background inflation rate. Therefore, although those journals appear to be increasing in quality as measured by the impact factor, they are actually failing to keep pace with inflation.
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Vol. 60 • No. 6