Méchante bonne question en effet. Un peu occupé, mais je retourne lire ton article sur la durée des albums d'ici la fin de la journée.
En attendant, y a cette note ci-dessous sur la certification qui explique bien des choses.
Y a plusieurs gros marchés (Japon, Royaume-Uni, France, Allemagne, etc.) pour qui on n'a pas de chiffres pour Elvis et les Beatles avant 1973. Et Elvis, qui dominait en 1956 et 1957, est aussi pénalisé sur le marché américain qui a commencé à certifier les ventes quelque part en 1958.
Certification systems have been established periodically throughout the past half century; thus, certification databases are not able to cover all sales. Some (or all) records released and sold prior to a certification system's establishment year may not be found within the available searchable certification databases. Year of establishment (from largest market to smallest based on Retail Value each market generates respectively):[8][9]
United States: 1958[421]
Japan: 1989[422]
Germany: 1975[13]
United Kingdom: 1973[423]
France: 1973[424]
Canada: 1975[425]
Australia: 1997[18] (online certification-database has begun since 1997. Certifications; however, existed in Australia since the 1970s)
Brazil: 1990[426]
Netherlands: 1978[21]
Italy: 2009[22] (online certification-database covers certifications issued since 2009. Certifications; however, have existed in Italy since the 1990s)
Spain: 1979[43]
Sweden: 1987[25]
Denmark: 2001[61] (online certifications for Denmark start from 2001. Certifications; however, existed in Denmark since the early 1990s[427])
Switzerland: 1989[428]
Mexico: 1999[429]
Argentina: 1980[430]
Belgium: 1995[431]
Austria: 1990[34]
Poland: 1995[35]
Finland: 1971[432]
Ireland: 2005[95]
New Zealand: July 1978[36]