The P/F ratio method is the most used indirect technique to estimate fertility in contexts of defective vital registration systems. This paper develops an analytical framework to evaluate how results of the application of the method are biased when one or more conditions of the method are not met, proposing adjustment factors to correct for these biases and to incorporate uncertainty in the estimates. Sensitivity analysis of indirect demographic methods, such as the P/F ratio, provides insights about the limitations of the methods and how sensitive they are to the violations of their assumptions and assist the estimates of the measures of uncertainty in demographic parameters. The paper proposes a sensitivity framework to the hypothesis of differential fertility between survivors and non-survivors; differential fertility between migrants and non- migrants; and fertility change.