The index, utilizing known quantities of labor and materials properly proportioned, has the advantage of being its own test for the base period. Because it has a fixed base, periodic surveys can be made to test whether or not the fixed base still reflects sewer construction practice. This method provides a yardstick to study the trends and magnitude of sewer project construction costs in constant dollar values. The two indexes--the Sewage Treatment Plant Construction Cost Index (PHS-STP) and the Sewer Construction Cost Index (PHS-S)-- when used in conjunction, provide the tools necessary to equate sewerage projects, one to another, in both time and place.