Patterns of citation densities by date of publication in physical review

Citations in Physical Review by the staff of the Physics Department of M.I.T. in 1970‐71 were analyzed as references‐permeter of shelf. These citation densities were independent of age for pre‐1962 volumes, but the mean density for the 1961‐55 volumes in the main library was twice that for less accessible 1954‐40 volumes in the basements. For the 1969‐62 volumes, the densities fell with age, perhaps partly due to the greater accessibility of the most recent personal copies, and partly to a difference between the relatively recent literature on the precise topics being discussed, compared with the less age‐dependent literatures on theory and methodology. There was no evidence that exponential models are to be preferred over linear models or that citation density patterns reflect library use.