Songs now stay in the UK's Top 40 singles chart for more weeks, on average, than at any point in the last 40 years. Of the 10 songs that have spent the longest time in the charts, nine come from the last decade. At the end of 2014, Happy by Pharrell Williams, had spent 49 weeks in the Top 40 – an all-time record.
This slowdown in chart turnover resembles a type of 'stagnation' in popular music. But unlike economic stagnation it does not necessarily carry the same negative connotations. The chart can't reflect the quality of music. What the slowdown does reflect, though, is the massive shift in how music is sold and consumed. The chart is unique in being able to capture this change, as it is both long-running (since the 1950s) and high-frequency (weekly).
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The data visualisation, above, (which requires a modern browser, such as Chrome,) shows a particularly marked change since the 1990s and early 2000s, when large numbers of new singles entered the chart each week. These singles tended to debut high and then fall quickly. Why? Some commentators have described how record labels tried to game the chart over this period. Their tactics included deep discounting, and issuing the same single in multiple formats to boost sales. The average time a single spent in the Top 40 in the 1990s and early 2000s was around four weeks.
In contrast, by the end of 2014, singles were spending an average of eight weeks in the chart. The rise began in 2005, and since then there has been a steady increase in the number of songs appearing in the Top 40 for at least 10 weeks. The start of this rise coincides with the Official UK Charts Company beginning to include downloads in its rankings.
The two events may be related. When music stores were physical spaces, shelf space was limited, and there were production and distribution costs associated with each additional sale. When a single began to slip in the charts and became less profitable, record companies would often 'delete' it in an effort to save money, to make room for new releases and to encourage album sales. When a single was deleted it was no longer eligible for the charts and this limited a song's potential run. The move to selling music online has increased shelf space and reduced the marginal cost of a single, allowing the big hits to stick around.
Another factor may be the 'blockbuster strategy' described by Anita Elberse. She posited that in the entertainment sector the most profitable strategy is often to make disproportionate investments in a few products: a small number of big hits. The digital revolution aids this strategy by broadening distribution channels and increasing economies of scale in marketing.
What does the increased staying power of big hits mean for the much-debated 'long-tail theory', which predicted that the move online would shift focus away from the hits and towards niche products? The theory reasons that moving online enables sellers to profitably stock niche products (found in the 'long tail'), and this leads consumers to reduce their demand for the big hits (found in the 'fat head').
The trend towards longer stays in the Top 40 does not preclude the long-tail theory, since the Top 40 only ranks the songs in the fat head of the distribution. However, the trend does suggest that certain forces may be countering the long-tail effect. Namely, if big hits are now sticking around, they may be occupying some of the extra shelf space (or prominence) that the long-tail theory presumed would be reserved for niche music. The increase in choice from the shift online may also be increasing consumers' reliance on cues – such as popularity – to guide their purchasing decisions. Both of these effects could reduce the long-tail.
Looking ahead, the slower turnover in the charts may be set to continue. In July last year the Official UK Charts Company began to take streamed music into account when compiling the chart, with 100 streams equating to one purchase. This change aids the shelf-life of songs and, although the full impact of streaming remains to be seen, turnover in the Top 40 has remained low since its introduction.
 This is a backward-looking, 12-month average to remove seasonal effects.
 These are the 10 songs which have appeared in the Top 40 singles chart for the greatest number of weeks (within a 10-year window) between 1975 and 2014 inclusive.
 Happy re-entered the charts in the second week of 2015 and has now spent 50 weeks in the Top 40.
 Downloads were initially incorporated in a restricted way in 2005. They were fully integrated into the charts in 2007.