Relative income mobility (a household's income change with respect to others) was also important, especially among the middle class (defined as those between 50% and 150% of the median income). Even if existing members of the middle dass did not move to the poor or rich groups, the majority changed their position within the middle dass itself (only 13% to 29% stayed within their initial dedle). One reason for this high within-dass mobility is that the large number of households in the middle dass narrows the values of each income dedle brackets, but also those at the bottom (top) of the income distribution can only stay in the same decile or move up (down).