There are two types of abstracts: informational and descriptive.
Informational abstracts
Communicate contents of reports
Include purpose, methods, scope, results, conclusions, and recommendations
Highlight essential points
Are short—from a paragraph to a page or two, depending upon the length of the report (10% or less of the report)
Allow readers to decide whether they want to read the report
Descriptive abstracts
Tell what the report contains
Include purpose, methods, scope, but NOT results, conclusions, and recommendations
Are always very short— usually under 100 words
Introduce subject to readers, who must then read the report to learn study results