The problem with extrapolating from old Chart-Track data

Chart-Track reported that PS3 sales increased by 999% last week after declining 20% the previous week. Whilst declining to release specific numbers they did say that the PS2 Slim had sold 21% more during it's release week in 2004. Chart-Track did release the actual PS2 Slim launch week numbers back then; 50,000.

Many sites, including MCV picked up on this, and calculated PS3 Slim first week sales at 40,000 (or 41,322 to be precise).

There are several problems with this though;

Firstly the 50,000 was almost certainly rounded, maybe by 5,000 or 10% if rounded to the nearest 10,000.

The second issue is where there is a bigger problem. Chart-Track's coverage has increased over the years, and with each increase in coverage their extrapolation model is adjusted and back-dated to reflect this. This can be seen in annual total market statistics for a certain year changing over time. Fortunately their market coverage has been around 90% since 2005, so any data since then shouldn't have been adjusted for this reason, but any data prior to 2005 is most likely obsolete and has since been adjusted.

PS2 launched in Q4 2004 when Chart-Track's coverage was 85%, but in Q1 2005 Chart-Track's coverage increased to 90% with the addition of Play.com, Sainsburys, John Lewis, Morrisons and Simply Games to their panel. The data from these stores which had previously been estimated would then have been applied to PS2 Slim's launch week sales, resulting in an adjustment to the originally reported figure. Unfortunately it's impossible to know by how much.

Chart-Track market coverage:



The third issue is similar to the second. As reported last month, even when their coverage hasn't changed Chart-Track adjusts it's extrapolation models based on consultations with the platform holders and back-dates the adjusted figures to the historical data. This resulted in Wii and DS being adjusted by roughly 5% recently.

When you add all of these together it creates quite a large margin of error, whilst PS3 may have sold 40,000 last week, it's quite possible that it sold significantly more or less than this. Unfortunately it's impossible to know an exact figure unless Chart-Track decide to release it.