Moreover, the ECB must always observe the principle of proportionality laid down in Article 5(4) of the Treaty on the European Union when exercising its mandate. Specifically, this implies that our measures need to be suitable, necessary and proportionate stricto sensu. •  For an instrument to be suitable, it needs to be able to address the respective risk to price stability.
•  For an instrument to be necessary, we must lack an alternative instrument, which could achieve the same result. •  For an instrument to be proportionate stricto sensu, its expected benefits should outweigh its costs. Let me apply these three criteria to one of the ECB’s most-debated instrument, namely the purchases of public-sector debt instruments, i.e.