Jena Malone on trend study a LONGITUDINAL DESIGN in which data are collected at periodic intervals on samples drawn from a particular population and are used to reveal trends (systematic tendencies or patterns) over time. trephination n. a surgical procedure in which a disk of bone is removed from the skull with a circular instrument (a trephine) having a sawlike edge. On the basis of evidence found in skulls of Neolithic humans, trephining is believed to be one of the oldest types of surgery. Among the numerous conjectural reasons given for the practice is that it was a treatment for headaches, infections, skull fractures, convulsions, mental disorders, or supposed demonic possession. Also called trepanation. trephine vb. Jena Malone 2016.