Dr Foster's ethics committee is an active regulatory body and is involved on a regular basis ('Experts query hospital rankings data', news, pages 4-5, 18 January).

The data's validity was questioned. It has been analysed independently at Imperial College under Professor Sir Brian Jarman's direction. His programme, spanning over 10 years, has examined 100 million patient episodes. His team's analysis demonstrates a strong statistical relationship between mortality and staffing levels.

You quote a Department of Health official indicating that one or two trusts had queries.

These relate to DoH data that we could not validate because trusts declined to help us. Where we are aware of errors, we will change them as quickly as we can. It is not the case that the DoH 'told' trusts to take part.

Officials encouraged them on the basis that our requests for information were reasonable.

Complaints can be addressed direct to me or Dr Jack Tinker, chair of the ethics committee.

Tim Kelsey Chief executive, Dr Foster 140 Tabernacle Street London EC2A 4SD