For the purposes of calculating sales, a single is currently defined by the Official Charts Company OCC as either a 'single bundle' having no more than four tracks and not lasting longer than 25 minutes or one digital audio track not longer than 15 minutes with a minimum sale price of 40 pence. Since September , the single has sold over 4. Sales of singles have been monitored and charted in the UK since , when Percy Dickins of New Musical Express NME telephoned around 20 record stores and aggregated their best-selling singles into a hit parade. The highest-selling by a solo female artist is " Believe " by Cher — released in October , it has sold over 1. Released in December , the single sold more than , copies in its first week, but was kept off number one by " Do They Know It's Christmas?