Audit and statistics

Details of statistical tests will not be covered here but general comments are required so that statistics are not abused when performing an audit. Descriptive statistics are useful and acceptable — distribution of ages, operation type, etc. Many surgeons will quote a ‘mean’ and ‘standard deviation’ for items such as age; this assumes that the data are normally distributed which is not necessarily the case. It is often better to use median and range for ages, time after surgery, etc. It is unlikely that incidence or prevalence can be calculated from an audit as the size of the overall population (the denom­inator) will not be known. The use of tests of significance is not acceptable in audit data. Performing tests of significance requires a comparison between two groups, one of which represents a control group. A control group is one which is identical to the test group except in the one aspect that is being examined. Therefore, tests of probability can only be used where patients are entered into a prospective randomised clinical trial.

The presentation of data in a publication is a source of concern in many audit papers. Often the scenario arises that a surgeon will find that a certain number of patients underwent a specific procedure some years before. The surgeon will then try and find the details from records of those patients but for various reasons it is only possible to find, for example, 75 per cent of the original records. The surgeon will next try and contact this 75 per cent of patients to find out the ‘outcome’ of the operation (usually using an invalidated questionnaire or score). Only, for example, 50 per cent of those contacted will reply to an enquiry (i.e. 50 per cent of 75 per cent of the original number). Some of the remainder may have died but many simply will not be traceable. In this process if the original number was 100, the assessment will be undertaken for only about

37 patients. Many authors will then make comments about the data that can only be applied to the 37 who were assessed but the report is written as if it refers to the total number.