Assessing the varying level of impact measurement accuracy as a function of the citation window length

With the passage of more time from the original date of publication, the measure of the impact of scientific works using subsequent citation counts becomes more accurate. However the measurement of individual and organizational research productivity should ideally refer to a period with closing date just prior to the evaluation exercise. Therefore it is necessary to compromise between accuracy and timeliness. This work attempts to provide an order of magnitude for the error in measurement that occurs with decreasing the time lapse between date of publication and citation count. The analysis is conducted by scientific discipline on the basis of publications indexed in the Thomson Reuters Italian National Citation Report.