The period studied was the six months immediately before and after the device change and, for this reason, most of the data collection was from retrospective written information such as patients' medical records and computer information.
(The period; studied; ) (The period studied; was; the six months; T:immediately before) (most of the data collection; was; from retrospective written information such as patients' medical records and computer information; T:after the device change)
The period The period studied most of the data collection
studied was was
the six months T:immediately before
from retrospective written information such as patients' medical records and computer information T:after the device change