One hugely important lesson branded into me from analyzing data is the importance of step-by-step procedures. This may sound elementary, but when you start with an Excel file of data from a client, it is crucially important to keep an audit trail of each step of your transformations and calculations.
If you change the names of columns so that they are consistent with code you have already written, you should record and store each change. If you add another variable [think of a variable as a column in Excel], you need to track how you made that addition. Do likewise for any calculations, such as calculating and storing external spending per lawyer. And, by the way, comments along the way complement your efforts to be logical and measured.
Data preparation always involves learning as you go, so if you haven’t saved the steps you have taken, you create nightmares of uncertainty about the quality of your data. Or you can’t figure out how you got (or failed to get) some result later on.