Measuring the degree of corporate tax burden is important for evaluating existing tax policies and setting the direction of new tax policies. Effective corporate tax rates have been widely used as an indicator of tax burden. The effective corporate tax rate is calculated by dividing corporate tax by income, and many studies have used various proxy variables to represent corporate tax and income. However, concerns have been raised about the inherent problems of effective corporate tax rates and the reliability of the proxy variables used to measure them. This paper examines the problems inherent in effective corporate tax rates from a theoretical perspective and through past empirical research results, and then directly confirms through empirical analysis that different definitions of effective corporate tax rates can lead to different research results. Six commonly used variables were selected as effective corporate tax rate measures, and we examined whether correlations exist among these proxy variables for listed companies on a year-by-year basis over a five-year period. The correlation coefficients showed some variation among proxy variables but generally exhibited low correlations overall. Similar results were obtained when calculating correlation coefficients for the five-year average effective corporate tax rates and for the entire sample over the five-year period. Furthermore, when the sample was divided into ten deciles based on effective corporate tax rate magnitude and examined whether each proxy variable assigned the same firm to different deciles, up to 53.2% of firms showed differences of three or more deciles, yielding results consistent with the foregoing analysis. Therefore, since the interpretation of a firm's tax burden can vary depending on which effective corporate tax rate proxy is used, any analysis employing effective corporate tax rates must verify whether research results differ according to the choice of proxy variable.