How to change the base year of a quarterly index [duplicate]

I have two indices with different base years. The first is car registrations which is an index with the base year of 2010. The other is harmonized index of car prices, with a base year of 2005.

The two are not comparable, so I need to change the base year of one of the variables to 2010. The problem is that the data is quarterly, so I'm not sure exactly how to proceed.

• Why should the fact that it is quarterly make a difference? – 410 gone Mar 13 '15 at 10:50
• Well the base year = 100. But the values I have are quarterly, not annual. So I don't know how exactly to change the base. With annual data values I'd just divide the index with the new base year value and multiply by 100, I'm not sure how to do that with quarterly values. – user3078258 Mar 13 '15 at 16:56
• So you're are interested by the level, not the growth of your series? Does the simple mean of your 4 points for 2010 and 2005 is exactly 100? – Yann Apr 28 '15 at 11:29
• I voted to leave open, since the OP needs help on what to do when the data are quarterly. It is helpful to clarify whether something different has to be done compared to the case of some other frequency, say yearly, or not. – Alecos Papadopoulos May 13 '15 at 0:47