The impact of aerosols on the climate and atmospheric environment depends on the water uptake ability of particles; namely, hygroscopic growth and activation into cloud condensation nuclei (CCN). The size-resolved activation ratios (SRAR), characterizing the fraction of aerosol particles that act as CCN at different particle sizes and supersaturations, can be measured using a combination of differential mobility analyzers (DMA) and particle counters. DMA-based measurements are influenced by the multiply charged particles and the quasi-mono-dispersed particles (effect of DMA transfer function) selected for each prescribed particle size. A theoretical study, assuming different particle number size distributions and hygroscopicity of aerosols, is performed to study the effects of the DMA transfer function and multiple charging on the measured SRAR and the derived hygroscopicity. Results show that the raw SRAR can be significantly skewed and hygroscopicity may be highly biased from the true value if the data are not corrected. The effect of the transfer function is relatively small and depends on the sample to sheath flow ratio. Multiply charged particles, however, can lead to large biases of the SRAR. These results emphasize that the inversion algorithm, which is used to correct the effects of the DMA transfer function and multiple charging, is necessary for accurate measurement of the SRAR.