Measuring a single scientist's output has always been problematic. Why? First, in order for the statistics to be reliable, the scientist has to produce a considerable publication output and get cited. That takes time. Second, measures like research productivity, number of publications and citations don't always correlates. Measuring the output of journals and universities has been far more reliable than measuring that of one person.
Suggested by h-index (2005) offers an attractive way of quantifying one's scientific output as a single number. The index is defined as:
Bornmann, L., & Daniel, H. (2008). The state of h index research. Is the h index the ideal way to measure research performance? EMBO reports, 10 (1), 2-6 DOI: 10.1038/embor.2008.233
Hirsch, J. (2005). An index to quantify an individual's scientific research output Proceedings of the National Academy of Sciences, 102 (46), 16569-16572 DOI: 10.1073/pnas.0507655102