Databases such as Thomson Reuters' ISI Web of Science, Scopus, Google Scholar and Microsoft's Academic Search allow authors to compute their own citation statistics, but they yield inconsistent results.
The discrepancies come from differences in information sources and in temporal citation coverage. Web of Science and Scopus, for example, provide citation data only for their indexed journals, giving different coverage for the number of journals, precursor articles and fields of academic research — often with regional biases (such as European versus US sources). Google Scholar includes all journals (indexed, free access and popular science), conference proceedings, books, theses, reports, local press and electronic sources — all subject to variable degrees of control and scrutiny.
A debate is crucial on how these tracking tools compare and should be used, given that their indiscriminate usage has potentially negative implications for academic careers.