The validity of international comparisons of labour productivity can be limited by a number of measurement issues. The comparability of output measures can be negatively affected by the use of different valuations, which define the inclusion of taxes, margins, and costs, or different deflation indexes, which turn current output into constant output.[5] Labor input can be biased by different methods used to estimate average hours[6] or different methodologies used to estimate employed persons.[7] In addition, for level comparisons of labor productivity, output needs to be converted into a common currency.