A week is long time in retailing, but makes no difference at HMV The switch from a ten-week comparison period to a nine-week period (for the first time since HMV started reporting on a quarterly basis in 2005) "made no difference" to the actual numbers posted by the troubled retailer insisted Fox, although the beleaguered chief executive admitted later that he hadn't actually worked out what the like-for-like sales decline would have been had they been calculated on a ten-week basis. With like-for-like retail sales down 9.8pc and total retail sales down 18.4pc, maybe the extra week was little more than academic. But HMV's decision to change its reporting period, highlights a fundamental problem with Christmas trading updates. Despite the City's obsession with like-for-likes sales as a measure, there is no definition, no specified accounting standard and no external audit. Chief executives less scrupulous than Fox can, and have, nudged like-for-like sales numbers in the right direction by changing the reporting period, including sales from extensions or booking items sold on promotion as full-priced sales. The reality is that not all like-for-like numbers are equal, which makes comparisons from one year to the next - let alone from one retailer to another - almost impossible.