Asking prices of the same models at the same shops, 13 Sep = 100.
Each day is compared with the day before on like-for-like pairs only: one model at one shop, listed on both days. The index moves by the average of those pairs’ changes, so a shop joining or a change in what is listed is not counted as a price change. Prices are converted at one rate for every day, and parts-only and brand-new units are left out.