In theory it is sufficient to rely on the certification of other QPs if the final certifying QP has knowledge of the other QPs and of the quality systems within which each of them is operating. In practice the final QP should be aware of any matter that might affect his/her decision to certify the batch (for example deviations or OOS results/investigations) and hence should review documentation time-to-time, particularly if contract manufacturers and contract QPs are involved in the process.