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I recall reading an article in which a psychologist written about a study that aimed to prove which method was more affective.

The conclusion of the study was that people are more likely to stick to a goal they keep to themselves. This is because often telling people a goal they will reward you with praise as if you already achieved it making it prematurely gratifying which causes you to lose motivation.

Also from my own experience public commitment is a bad motivational tool. This is because the motivator of public commitment is usually fear (failure etc) and fear is a terrible mindset to draw motivation from which (in my experience) can cause a lot of procrastination.