The ACI Building Code criteria for estimating the required average strength f'cr is evaluated using actual field data. Basically, the current criteria specify that the required average strength f'cr should be larger than the specified compressive strength f'c by a certain amount. The magnitude of overdesign or the difference between f'cr and f'c would depend on the quality control of the particular concrete suppier. The quality control, which is related to the variability of the strengths over a period of time, is usually measured in terms of standard deviation. The code specifies that the amount of overdesign should be based on the standard deviation obtained using a minimum number of specimens and a multiplying factor t. The factor t is chosen to result in no more than 1 in 100 low tests. The code equation is consistent with the recommendations of Committee 214, Strength Evaluation. Committee 214 also recommends a set of t values that can be used for any specified permissible low tests, such as 1.28 for 1 in 10 low tests. The numerical values of t are based on the assumption that the compressive strengths of a large population of specimens follow the normal frequency distribution curve. This basic assumption is evaluated in this paper using the field test data obtained from various geographic locations. The average compressive strength varied from 3,512 psi to 11,057 psi (24.3 MPa to 76.2 MPa). Quality control, measured in terms of standard deviation, varied from excellant to poor. The results show that the values of t obtained using normal distribution are generally conservative, but there is a variation in the occurrence of low tests. The field data is used to set the upper and lower limits of low tests for a given t. Modification of ACI Building Code equations is proposed for a more conservative estimate of the required average strength f'cr.