In Journal Citation Reports, the quartile rankings are defined based on rank for the Journal Impact Factor. In Journal Citation Reports, quartiles are defined as the following:
Q1: 0% < Z ≤ 25% Highest ranked journals in a category
Q2: 25% < Z ≤ 50%
Q3: 50% < Z ≤ 75%
Q4: 75% < Z Lowest ranked journals in a category
Z is defined as:
Where X is the journal rank in category and Y is the number of journals in the category.
When sorted by Impact Factor, if a journal is rank 78 out of 314 in a category, Z=(78/314)=0.248 which is a Q1 journal.
When sorted by Impact Factor, if a journal is rank 102 out of 204 in a category, Z=(102/204)=0.5 which is a Q2 journal.