  The number of outcomes is a defining property of a quantum measurement, in
particular, if the measurement cannot be decomposed into simpler measurements
with fewer outcomes. Importantly, the number of outcomes of a quantum
measurement can be irreducibly higher than the dimension of the system. The
certification of this property is possible in a semi-device-independent way
either based on a Bell-like scenario or by utilizing the simpler
prepare-and-measure scenario. Here we show that in the latter scenario the
minimal scheme for a certifying an irreducible three-outcome qubit measurement
requires three state preparations and only two measurements and we provide
experimentally feasible examples for this minimal certification scheme. We also
discuss the dimension assumption characteristic to the semi-device-independent
approach and to which extend it can be mitigated.
