Concerned with the validity of the audience data reported by two major syndicated magazine readership services--Mediamark Research Inc. (MRI) and Simmons Market Research Bureau (SMRB)--a study examined the differences in MRI and SMRB calculations of reader turnover rates, which form the basis for estimating audience accumulation. Results indicated that the MRI recent reading method of determining audience accumulation tended to produce significantly lower turnover levels than the SMRB two-interview method, but that the differences were contingent on circulation size. MRI turnover levels, for example, were no different than those provided by SMRB for publications with circulations under 500,000. The reason for this finding may have been based on complex interactions of the proportion of in-home to out-of-home readers, the circulation size, and the frequency of reading measure. Since SMRB and MRI defined readership differently, the findings did not indicate which method of determining audience accumulation was most effective. (MM)