A sentence must have at minimum a subject and a verb; it also usually contains modifiers. Sentence fragments, which lack a subject or a verb, should not be used in scientific or technical writing (except within the structured abstract; see 2.5, Manuscript Preparation, Abstract). Occasionally, writers of prose and poetry use sentence fragments intentionally, for effect.
Her affect signaled depression. Utter depression.
In scientific writing, these fragments are likely to be unintentional and are definitely inappropriate.
The clinical spectrum of disease varying according to the population and age group under study.
The clinical spectrum of disease varies according to the population and age group under study.