1. To properly compare albums from different years, this countdown will looks at an album's sales in relation to the total volume of album sales that year. The higher an album's market share, the higher it will rank. For example:
Album
X: a 1999 album with 10M sales has a 10M/3183M = 0.00314 = 0.314% unit market share
Album
Y: a 2012 album with 8M sales has a 8M/1062M = 0.00753 = 0.753% unit market share
Y will rank higher than
X because it has a higher market share, which indicates that
Y's domination during its time was (2.40×) bigger than
X's. It's a neutral way to compare albums from different years.
2. The score for an album will be its market share, expressed in terms of 1999 sales. For album
Y : the adjustment factor = 3.00 as it was 3.00× harder to sell an album in 2012 compared to 1999 (see table above).
Y's score would be 3.00*8M = 24M.
The score reflects that selling 8M in 2012 is equally impressive as selling 24M in 1999.
3. A similar thing is done in the music industry: France adjusted the platinum threshold from 400k to 100k to indicate that selling 100k now is equally impressive as selling 400k back then due to the market decline. Same with platinum thresholds in Canada (100k->80k), Germany (500k->200k), Japan (400k->250k), etc.
4. Using just sales to compare which album is 'bigger' would be meaningless as it'd be completely lopsided towards early albums due to piracy/streaming (see table above). The countdown would reflect "albums of 1999-2014 that were released before 2006" rather than actually reflect the biggest albums and most dominating albums of 1999-2014.
5. Most sales estimates of pre-2010 albums are from MJDangerous. Most sales estimates of newer albums and some old albums are from mediatraffic/chartsinfrance. Also credit to IFPI for WW yearly figures and Kworb for the methodology.
It kicked a pop girl album out of the top 100
