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Dear Colleagues, I have received the following query from Tim Blackburn ([log in to unmask]) If anyone could help him I am sure he would be most grateful. All good wishes, Olive I wondered if I might presume to ask for your advice on how to classify the Oxford level of evidence for my paper. I am submitting a paper on a non-concurrent (before-and-after study) cohort study with 13% loss to follow up. The cohorts are consecutively recruited but the 'before' cohort was retrospective and the 'after' one was prospectively studied in a non-blinded way. Would you classify this as level 2b, or 4 due to lack of identical outcome measurement (i.e. retrospective and prospective) using the Oxford system? The primary outcome measures are hard objective data - pre-op and postop haemoglobin measurements plus numbers of units of blood transfused in the interim (from which a bleeding index is then derived). I am seeking an independent expert in literature reviewing such as yourself who may be familiar with the Oxford CEBM levels of evidence system. I will fully understand if you are too busy to reply to this presumption on your time. Warm regards Tim Blackburn FRCS(OMFS)(Ed) FRCS(Clin Surg in Gen)(Ed) FDSRCS(Eng) MBBS(Hons)(Lon) BDS(Hons)(Lon) Consultant Oral Maxillofacial Head & Neck Reconstructive Surgeon, Manchester Royal Infirmary T: +44 (0)7885 294097