It also helps to actually go back and look at in terms of "rejecting the null hypothesis". If you're talking about a treatment for The Common Cold, the null hy

It also helps to actually go back and look at in terms of "rejecting the null hypothesis". If you're talking about a treatment for The Common Cold, the null hypothesis is "the subject got better after awhile because people get better after awhile", and you can't disprove that's what's happening without a very rigorous study with a well designed control. If you're talking about "here's some robot eyes that cure blindness", you don't really need a control group to test if it works. The null hypothesis is they're blind; you just need to demonstrate they can see to disprove the null hypothesis and prove efficacy.