I NDICES of the cost of constructing a standard reinforced concrete factory building, supplied by the Aberthaw Construction Company of Boston to the REVIEW, are shown in the accompanying table. The basic figure is the actual cost of construction of a sevenstory reinforced concrete factory building erected by the Aberthaw Company in I9I4. Estimates have been made of the cost of putting up the same building if construction were begun on January I, I920, July I, I920, and on the first of the month from January to October I92I. The index number for any given date is secured by expressing the estimated cost of construction, beginning on that date, as a percentage of the actual cost in I914. Although the indices are based on the estimated costs of constructing a selected type of building they show the fluctuation in costs of reinforced concrete construction in general. The building chosen as a base was actually constructed between August 22 and December 31 I9I4; building costs were so uniform during the year that it is accurate to consider the base to be the entire year. This b,uilding was chosen for two reasons. The building itself is typical of the reinforced concrete structures erected by the company, as is also the proportion of costs involved for the various classes of labor and various kinds of materials. In the second place, its construction was an almost ideally handled job ; that is, a job which involved no waste and no unusual costs. Consequently, actual and estimated costs correspond closely, both for the job as a whole, and for its component parts.