6. A non-transitory computer-readable medium comprising program instructions, which, when executed by a processor, cause the processor to perform a method for calculating a normalized activity score to rank an identified document, the method comprising: identifying a stored document; determining a number of times the identified document was cited in a subject matter community of the identified document; determining a probability distribution that individual documents within the subject matter community are cited a variable number of times by other individual documents in the subject matter community; calculating a probability function by performing a regression on the probability distribution; calculating the activity score value according to an activity score function formulated as an inverse of the probability function such that the activity score function is defined by: Score ⁡ ( x ) = k ( a · x α + 1 ) p , wherein: Score(x) is the activity score value, k and p are constants, x is the number of documents citing the identified document, and a and α are learned from the regression on the probability distribution; and the activity score function is such that the activity score value is calculated according to a probability that the individual document in the subject matter community is cited a number of times greater than or equal to the number of times the identified document was cited in the subject matter community; weighing the activity score value by an age of the identified document; and storing in computer memory a ranking of the identified document based on the activity score value.
8. The non-transitory computer-readable medium of claim 6 , wherein the calculating the activity score further comprises: generating an activity score vector; calculating an individual activity score value for dimensions of the activity score vector, wherein at least one of the dimensions is associated with the number of times the identified document was cited in a subject matter community of the identified document; weighing at least one of the individual activity score values by an age of the identified document; and adding the individual activity score values to obtain a total activity score value.