Age validation is commonly based on the marginal increment ratio (MIR). This procedure has been used to validate the annual periodicity of growth band deposition. In this study, MIR data from geoducks Panopea globosa and Panopea generosa were analyzed using a new method for age validation based on a sinusoidal model. The model used four parameters, including the amplitude |δ|, defined as half the distance between the highest and the lowest values of the function; ρ, which is the cycle from 0–2π; τ, which is the horizontal shift; and λ, which is the vertical shift, or displacement. When the parameters in the sinusoidal function were estimated, the period could be computed as . The parameters were estimated using a maximum likelihood estimator. In addition, temperature and chlorophyll a were analyzed relative to the individual growth in shells. Based on parametric and nonparametric tests of the MIR data, the analysis identified a monthly progression of growth increment formation over an annual period. The data indicated that the shell growth of the outermost band is synchronized in both species; however, analysis of variance and the Kruskal—Wallis test applied to P. generosa and P. globosa, respectively, could not describe the individual growth as an annual sinusoidal cycle. Similar results were found with the environmental variables, which suggested seasonal changes in environmental variables are associated with measurements of the MIR. The sinusoidal model proposed in this study confirmed the annual cycle of growth lines for P. generosa (Ω = 12.16 mo; CI, 11.91–12.53 mo; P<0.05) and P. globosa (Ω = 11.82 mo; CI, 11.52–12.16 mo; P<0.05). The model provides reasonable fits to the data and can be used for age validation in both species.