While Jim Simons is a mathematician and Rentec clearly has hired many brilliant people with PhDs, it's maybe worth mentioning the actual mathematics being used in their work isn't super high level difficult, impossible or secretive. Many of the PhD's working there do not have a PhD in math, but rather something like Physics, so I would say if you are familiar with graduate level math courses you can understand the math needed for this type of work. Math isn't where their edge comes from. Also Medallion is 30 years old, their early work in the mid 80s was done on computers with less processing power than your phone, "Machine Learning" as the term is being used lately or access to supercomputing hardware no one else knows about is also not where their edge came from.
They need smart people, but hiring the smartest people and having the most sophisticated models won't do you any good if you can't acquire high signal data, can't clean that data properly and can't rapidly backtest. And if you can't do any of that, adding more data is just going to add more noise.
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By the way for anyone reading this, Rentec is hiring, need to know C/C++ and a "basic knowledge of statistics" is preferred but not required.
Yes.
> Is their advantage in finding signals in data that other people overlook, or finding new data sources?
Yes.
I won't go into any particular detail, but there is a lot of signal in the market for those who are imaginative. The obvious and low hanging fruit is long gone, but there are still many places that offer an edge.
The industry is also extremely secretive (necessarily so). You'll hardly ever find a good treatise on finding novel signals, but there are tutorials on algo trading in general with examples of production strategies that used to work which have been, as you say, traded away. For that purpose I'd recommend you start here: http://www.decal.org/file/2945
In other words, Rentec is not just pointing machine learning models at data, they're investing in a very robust data processing pipeline. Everything before the analysis is just as important as the analysis at funds like theirs.