Deciding which of two bioevents is the less diachronous is a common problem in biostratigraphy. The most accurate correlation uses the finest timescale available. Chemostratigraphy or cyclostratigraphy offer a potential precision of about 10 ka. Graphic correlation can then be used to test the precision of bioevents and to quantify any mismatch. It can also be used to determine in which section any event occurs earlier. Application of these ideas to correlation of the Cenomanian–Turonian and Coniacian–Santonian boundaries demonstrates that some bioevents are as precise as chemo- and cyclostratigraphy, but that most are not. Two problems occur with bioevents. First they may not be recognizable in all sections. Second, where they are recognizable, they may be diachronous. In the former case, calculating confidence intervals on known ranges in sections where the relevant fossil has been recorded is an alternative test. Large confidence intervals suggest that both first and last occurrences of a fossil may be diachronous bioevents. At the Cenomanian–Turonian boundary the following bioevents (in stratigraphic order) appear to be reliable time planes for international correlation. The last occurrences of (1) Corolithion kennedyi, (2) Rotalipora greenhornensis, (3) Axopodorhabdus albianus, (4) Rotalipora cushmani, (5) Lithraphidites acutus, (6) Microstaurus chiastius and (7) the first occurrence of Quadrum gartneri. At the Coniacian–Santonian boundary only the first and last occurrences of Platyceramus undulatoplicatus, and the first occurrences of Platyceramus cycloides and Lucianorhabdus cayeuxii have been identified as potentially reliable bioevents.