Songwriter and record producer Kara DioGuardi was added as a fourth judge on the panel. She had previously collaborated with Celine Dion, Hilary Duff, Britney Spears, Enrique Iglesias, Leona Lewis and Christina Aguilera, and produced several Top 40 hits. As a songwriter, she had already worked with several American Idol alumni and winners, including Kelly Clarkson, Carrie Underwood, David Archuleta and David Cook. Meanwhile, Ken Warwick became the new executive producer, as Nigel Lythgoe had left the show to focus on "So You Think You Can Dance" and his new show with Simon Fuller, "Superstars of Dance".