The top two panels show the number of quasars with C iv BAL absorption and the number with C iv BAL variability at each velocity. The third panel is the second panel divided by the top one, which gives the fraction of C iv BALs that varied at each velocity. The dashed, dotted and solid curves indicate results for the long-term, short-term and combined data set comparisons, respectively. The bottom panel displays the combined data set with 1σ error bars.