The correlation between the progressivity index based on tax schedules (for single taxpayers and under the assumption of a similar distribution of income across countries) and the Kakwani progressivity index based on household surveys is 0.58, significant at 5%. Due to the various tax reliefs for low-income earners with children, the US personal income tax is much more progressive for families with children than for single tax payers. The Slovak Republic replaced the progressive personal income tax system by a flat rate system in 2004.