On completion of data entry, a sample of 100 of the complete LBC1936 clinic visit packs (containing demographic, cognitive testing and medical data information) were checked in their entirety against their respective entries in the SPSS database to ensure accuracy of data entry. We found insufficient errors in the sample of 100 to warrant a complete data entry check. All have been kept in a data log book. There were errors of entry in 30 out of 22,400 cells (0.13%; 100 subjects each with 224 columns of data). Errors were mostly small, numerical errors in cognitive test scores, blood results and medical data (e.g. height and lung function). In several cases the error occurred after a decimal point. There were no systematic errors. All errors and discrepancies found were noted in a log, checked and amended on the SPSS database.
