Rising publication delays inflate journal impact factors.

There is a slight trend for decrease in the lag-corrected difference after outliers are removed, but this does not reach statistical significance (p = 0.12, paired t - test).

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While this effect is highly significant, it could be attributed to the fact that citations occurring before print publication (i.e. within the publication lag) are not considered in our analysis – while they do count for the ISI impact factors, albeit only in the numerator, as described in the Methods section and in [16] .

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