Temporal Changes in the Parameters of Statistical Distribution of Journal Impact Factor

The statistical distribution of Journal Impact Factor (JIF) is characteristically asymmetric and non-mesokurtic. Even the distribution of log10(JIF) exhibits conspicuous skewness and non-mesokurtocity. This paper estimates the parameters of Johnson SU distribution fitting to the log10(JIF) data for 10 years, 1999-2008, and studies the temporal variations in those estimated parameters. The ‘overthe-samples stability’ in the estimated parameters for each year is also studied by the method of resampling close to bootstrapping. It has been found that log10(JIF) is Pearson Type-IV distributed. Johnson SU distribution fits very well to the data and yields parameters stable over the samples. The paper concludes that Johnson SU distribution is the best choice to fit to the log10(JIF) data. It is also found that over the years the log10(JIF) distribution has become more skewed and leptokurtic, possibly suggesting the Matthew effect in operation, which means that more cited journals are cited even more over time