This is where I record an assortment of (generally quantitative) techniques for research and problem solving that I've picked up over the years and have found to be useful in various domains.
I tend to forget the details of this stuff when I go for a while without using it, so outlining them here helps me solidify my understanding and improve my eventual recall, while also providing a one-stop reference that makes it quicker for me to pick something back up after I've been away from it.
Right now most of the pages here are more "notes to self" about specific topics than detailed references - over time the goal is to fill in the blanks with deeper content to better solidify my understanding in the various areas.