What technical analysis is, and is not

A rule was chosen because it had the best results on 2015 to 2020 data. Which test tells you most about whether it has an edge?

• Its results on 2015 to 2020 again, with a longer chart
• Its results on 2021 to 2024, data it was never chosen or fitted on
• The number of people who shared it

Which is the clearest example of data snooping?

• Testing one rule, written in advance, on ten years of data
• Testing thousands of variants and reporting only the one that did best
• Including trading fees in a backtest