  The Born rule assigns a probability to any possible outcome of a quantum
measurement, but leaves open the question how these probabilities are to be
interpreted and, in particular, how they relate to the outcome observed in an
actual experiment. We propose to avoid this question by replacing the Born rule
with two non-probabilistic postulates: (i) the projector associated to the
observed outcome must have a positive overlap with the state of the measured
system; (ii) statements about observed outcomes are robust, that is, remain
valid under small perturbations of the state. We show that the two postulates
suffice to retrieve the interpretations of the Born rule that are commonly used
for analysing experimental data.
