I'd say: 1. You need research skills that will allow you to ask the right questions, define the problem and put it in a mathematical framework. 2. Familiarity

I'd say: 1. You need research skills that will allow you to ask the right questions, define the problem and put it in a mathematical framework. 2. Familiarity with math (which? depends on what you are doing) to the point where you can read articles that may have a solution to your problem and the ability to propose changes, creating proprietary algorithms. 3. Some scripting language (Python, R, w/e) 4. (optional) Software Engineering skills. Can you put your model into production? Will your algorithm scale? Etc.