The number of cited fields, the Pratt's measure for the number of cited articles of each field, and mean citation age (i.e., the variety, concentration, and recency of cited articles) are considered to be strongly related not only to the interdisciplinarity of researches but also to their subjects. However, in comparison within areas in the same subject, it was confirmed that, in the area whose interdisciplinarity is regarded to be high, the number of cited fields is larger while the Pratt's measure is lower. Further, even in the areas where the numbers of cited fields are nearly equal, there appeared a difference in the Pratt's measure. In identification of interdisciplinary areas, indices measuring the concentration such as the Pratt's measure are suggested to be useful.
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