Fault Tree Analysis( FTA ) Vs Event Tree Analysis(ETA): Which one to choose?

It is easy to get confused between these two techniques. Indeed, the two are in fact complimentary (and are often used together) but focus on opposite sides of an undesired event.
This is sometimes called a ‘bow-tie’ model (because it looks like one) and when complimentary FTAs and ETAs are used, it’s called the bow-tie technique. The diagram only shows a single ‘undesired event’; in reality, multiple causes can lead to many different events initially, each then escalating with multiple consequences. You can analyse each event with FTA and ETA. In summary, FTA is concerned with analysing faults which might lead to an event, whereas ETA is interested in stopping it escalating. Both can be applied qualitatively or, if you have the data, quantitatively.