IEC/TR 61282-10:2013(E) defines the error vector magnitude (EVM) as a metric for quantifying the quality of an optical vector-modulated (modulation of phase and possibly magnitude) signal from a transmitter or optical transmission link. The considerations required for reproducible measurement results are detailed. The relationships with other related parameters from constellation diagram analysis like error vector, phase error, magnitude error, I-Q offset and time-resolved EVM are described, as well as the relationship between EVM and Q-factor. Key words: error vector magnitude (EVM), quality of an optical vector-modulated signal