When you think about your hypotheses you will often come up with a very long list. Which one should you tackle first? That's where the riskiest assumption test (RAT) comes in.
The RAT is the assumption that, if proven wrong, will make the whole venture fail. A lot of people define the RAT in a broad term that requires building the venture to test it (and it takes months if not years). That is the wrong approach. The right approach is to write out all your hypotheses, see which one is the riskiest, then see what you can do about deconstructing that RAT into its components and then see which component is the riskiest... and rinse and repeat.
Hackernoon has a great article that discusses this methodology. Read on to learn more.