The common difference between this series of numbers is 6.5. In other words 9-2.5 is 6.5, 2.5 and -4 shows a common difference of 6.5, -4 and -10.5 difference is still 6.5 (-6.5) and -10.5 t0 -17 is -6.5 so even where there are plus and minus numbers they are such that the absolute difference is always 6.5.
