Antimicrobial resistance is an emerging public-health threat. Studies of the relationship between antibiotic use and resistance, as well as surveillance programmes, examine changes in the proportion of isolates that are resistant. Although proportions are helpful to the clinician prescribing empirical therapy, proportion-based analyses can be misleading to the public-health professional as they can yield biased estimates. Proportions do not adequately reflect the burden of resistance, a measure often of interest in public health. A more appropriate measure of this burden is the rate of isolation of resistant organisms, that is, the absolute number of resistant isolates in a population over time.