Further, the history of popular music shows nearly as many remarkable failures to chart as it does impressive charting histories. Certain artists (such as Pink Floyd and Led Zeppelin) had tremendous album sales while being oblivious to the weekly singles charts. Business changes in the industry also affect artists' statistical "records." Single releases were more frequent and steady, and were expected to have much shorter shelf lives in earlier decades, making direct historical comparisons somewhat specious. Of the sixteen singles to top the Billboard chart for more than ten weeks since 1955, just one was released before 1992. During the first forty years of the rock era, no song had ever debuted at number one; since a 1995 change in methodology, nineteen songs have.