These comments generally read to me along the lines of: "It's totally reasonable to roll your own cryptography, as long as you're an expert in the field and do

These comments generally read to me along the lines of: "It's totally reasonable to roll your own cryptography, as long as you're an expert in the field and do it correctly". The rules of "Don't Stop And Peek" is general advice that is given out because the vast majority of people that conduct these kinds of trials are not statisticians, and will not do the math. Those that attempt the math will likely do the math wrong. So, you're totally correct that if you know exactly what you're doing as a statistician and apply the math correctly, it can be possible to peek at results while still drawing a conclusion. This ignores the fact that, for most humans who are receiving the advice, this is terrible advice. So, general rule: "Don't Stop and Peek". Advanced rule: "Unless you are a trained statistician, Don't Stop and Peek".