Slovakia expects liberal reform of tax system next year. Probably the most visible turn represents implementation of ,flat tax", which withal means broad elimination of personal income tax progressivity. What effects brought last decade of ,,classic" personal income tax with its progressive scale of marginal taxes, tax brackets and tax allowances? This paper explores tax progressivity, the first part is aimed at theoretical terminations; and approaches to a tax progressivity, in the second part authors deals real estimates of income tax progressivity - employees progressivity - in Slovakia in years 1993 - 2002. Tax progressivity is investigated in two variants (with or without social and health insurance payments), because of similar incidence of obligatory insurance with personal income tax. Tax progressivity is analysed for. 3 types of taxpayers in regard of a size of tax allowances (taxpayer allowance, child allowance). Recency of this paper can be seen in elected technique of computation for interval progressivity where variable limits of investigated intervals are used. The theory of taxation discerns between local. a global progressivity. Local (in particular interval) progressivity measures changes in effective rate in one point or between two points of an income scale and is closely adherent to an effective taxation. Subject publications detail 3 ways of measuring interval progressivity: average rate progression, liability progression and residual income progression. Average rate of progression measures the ratio of change in effective rate to change in income, liability progression represents the ratio of percentage change in liability to percentage. change in income and residual income progression Measures the ratio of percentage change in after-tax income to percentage change in before-tax income. For practical requirements of economic policy is attention of economist aim at local progressivity estimation. The measures of global progressivity are aimed at the Lorenz method of income distribution analysis. Among the most popular in measures rank the Lorenz curve and the 'Gini measure, Musgrave-Thin index, Kakwani index, Suits index, Atkinson measure, Robin Flood index and Theil index. The second part of this paper turn to concrete estimations of interval progressivity taking an example by employee in Slovakia in years 1993 - 2002. Progressivity Was computed for 8 ranks of nominal wages. For better predictability were 3 types of taxpayers with variant social considered: taxpayer A claiming to basic tax allowance, taxpayer B claiming to basic tax allowance and into the bargain allowance for I child, taxpayer C with basic tax allowance and allowance for 2 children. Enumerations were realized in excel program environment gain values of progressivity were ranged into 8 tables which contain, 1740 numerical values. Computed values offers wide range of variant interpretations and particular conclusions: . tax progressivity indicators comparison in the event of social and health insurance inclusion . interval progressivity development in a relationship with taxpayer's social status . development of an individual income tax progressivity changes . wage and legislative factors influence upon tax liability . interval progressivity and efficient taxation relationship. The analysis of an individual income tax progressivity in Slovakia 1993 - 2002 detected several facts and there is some possibility to formulate certain recommendations for further analysis of a tax incidence (a theoretical conclusion of the analysis) and to characterize progressivity of an individual income tax in Slovakia (a practical conclusion of the analysis). The analysis detected broad insensitivity of the ratio of change in effective rate to change in income. Contrariwise the ratio of percentage change in liability to percentage change in income and the ratio of percentage change in after-tax income to percentage change in before-tax income response to on-going changes very sensitively. Both mark analogical process. In consideration of less sensitivity there is no reason to apply the ratio of change in effective rate to change in income. Farther finding was option to brake away from social and health insurance payments. Computations of a tax progressivity in a case of social and health insurance inclusion and for a case without these insurance payments lead toward similar results, for the ratio of change in effective rate to change in income are the results even identical. Neither changes in magnitudes of tax allowances nor their size had any essential influence on tax progressivity changes. Tax progressivity was affected - mainly in years 1993 - 1999, by average wage development. Later on - since 2000 - tax progressivity was determined chiefly by changes in a tax-legislation (indexing of tax brackets and tax rates in them). Effective tax rates compared with the ratios of interval progressivity imply that solitary growth of effective tax incidence had no fundamental influence on tax progressivity changes. Empirical computations verified that the index of tax incidence (effective tax rate) is static quantity on the other hand tax progressivity was investigated as a ,,stream" -quantity. This is one of the reasons why is not possible to replace these quantities. Probably there is no explicit relationship between them. In elected interval was remarked the most significant growth of tax progressivity for the interval I - 3 multiple of average wage, further for all types of tax-payers in income-. interval of 5 - 10 multiple of average wage. Tax burden had been rolling not only to high-income groups of tax-payers in these years but to those so-called,middle-class" as well. Values of effective taxation show continuous growth in years 1993 - 1999 which was caused by advance of an average wage. Lately in 2000 less tax incidence was succeeded however labour taxation in 2002 is still higher than in 1993. Though the changes in progressivity are probably not perceived by tax-payers, those can contribute to destimulation of working effort or to higher rate of substitution between work and leisure. Results of tax progressivity measuring should be counted at every decision-making in an event of tax legislation constitution.
