The ARIA Singles Chart ranks the best-performing singles in Australia. Its data, published by the Australian Recording Industry Association, is based collectively on each single's weekly physical and digital sales. In 2008, fifteen singles claimed the top spot, including Timbaland's "Apologize", which started its peak position in late 2007. Twelve acts achieved their first number-one single in Australia, either as a lead or featured artist: Leona Lewis, Flo Rida, T-Pain, Colbie Caillat, Gabriella Cilmi, Jordin Sparks, Katy Perry, Kid Rock, Lady Gaga, Colby O'Donis, Kings of Leon and Wes Carr. Five collaborations topped the chart. Lady Gaga earned two number-one singles during the year for "Just Dance" and "Poker Face".