And that made it much more difficult to implement a single monetary policy. So once again the ECB had to step in to ensure that monetary policy remained effective. It began to buy government bonds through its Securities Markets Programme, SMP for short. For a while the SMP helped to rein in uncontrolled surges in government bond yields.
We cannot mechanically follow the rate path provided by our models because there is simply too much uncertainty in the world. We consider it misleading to pretend that uncertainty does not exist. There is always a degree of uncertainty when using economic models, but these days there is a litany of things we simply do not know.