What subject do you find most interesting?

Collective Intelligence and how to scale it to millions of people.

The first project I found that actually scales is: https://pol.is

https://en.wikipedia.org/wiki/Collective_intelligence

Time series analysis, both univariate and multivariate, and modeling not just the conditional mean but the conditional co(variance). There is a rich literature of linear models used by statisticians and economists (ARMA, VAR, cointegration, GARCH) and also a machine learning literature (RNN, LSTM) that I know less about.

Ethics