Based on Impact Factor (IF) data, the Journal Citation Reports published by Thomson Reuters (see also here) provides yearly rankings of science and social science journals, in the subject categories relevant for the journal (in fact, there may be more than one).
Quartile rankings are therefore derived for each journal in each of its subject categories according to which quartile of the IF distribution the journal occupies for that subject category. Q1 denotes the top 25% of the IF distribution, Q2 for middle-high position (between top 50% and top 25%), Q3 middle-low position (top 75% to top 50%), and Q4 the lowest position (bottom 25% of the IF distribution). For example, the 2009 Impact Factor for the ACM TRANSACTIONS ON SENSOR NETWORKS is 1.938. Thus, the journal ranks 32-nd (out of 116 journals, Q2 quartile) in the subject category COMPUTER SCIENCE, INFORMATION SYSTEMS, while it ranks 11-th (out of 76 journals, Q1 quartile) in the subject category TELECOMMUNICATIONS.
Unfortunately, papers cannot be easily associated to a single ISI subject category (at least, not always), and one has therefore to consider the full range of quartile rankings of the journal. Following this line, a quartile score (indeed, a discrete distribution) is associated to any paper published in IF-ranked journals by uniformly distributing a unitary mass over the quartile rankings of the journal
in which the paper was published (for that year). So, for example, papers published in 2009 in the IEEE TRANSACTIONS ON ROBOTICS are given a quartile score of [Q1=1, Q2=0, Q3=0, Q4=0], since in the (unique) subject category ROBOTICS the journal ranks 2-nd out of 16 journals. On the other hand, a paper published in 2008 in COMPUTER METHODS AND PROGRAMS IN BIOMEDICINE would be given a quartile score of [Q1=0, Q2=1/2, Q3=1/4, Q4=1/4], since the journal ranks 41-st out of 94 journals (Q2) in COMPUTER SCIENCE, INTERDISCIPLINARY APPLICATIONS, 35-th out of 84 (Q2), in COMPUTER SCIENCE, THEORY & METHODS, 40-th out of 52 (Q4) in ENGINEERING, BIOMEDICAL, and 14-th out of 20 journals (Q3) in MEDICAL INFORMATICS.
Quartile scores can finally be cumulated for all the papers published in a given year, providing synthetic views such those reported here.