First, several important cardiovascular risk factors, such as smoking habit, body mass index, physical activity, psychologic stress and family history of PAOD were not available in the NHIRD. Second, the diagnoses of cataract, PAOD, and other medical comorbidity were totally based on the ICD codes, and one might challenge the accuracy of this dataset. However, a thorough mechanism to confirm the accuracy and reliability of this dataset has been well established and propensity score matching was used to minimize the study bias [25]. Finally, the severity of cataract, PAOD and other medical comorbidities were uncertain in the NHIRD.