Transfer fees are not always officially confirmed by the transacting clubs, and figures published by unofficial sources may or may not take into account various fees such as those paid to agents or a third party, performance-related elements of the fee, and the notional value of any players included in part exchange. This leads to different figures being given by different sources. Performance-related clauses have become more common in recent years, meaning that it is harder to produce definitive lists of the largest transfer fees than was the case in the past. For example, the 2004 transfer of Wayne Rooney from Everton to Manchester United cost an initial fee of £20 million that rose to £27 million due to the number of appearances he made and the number of trophies he won with the club.