Reduce Multidimensional Data
After showing many thread arc versions, Kerr says that when you are dealing with multidimensional data, pick two variables; otherwise, you're going to end up with a big mess. He says this literally, but don't forget that you can also reduce dimensionality with super special and magical statistical methods.
Use Real Data
You won't know what you're really dealing with until you have the real data. You can spend lots of time guessing what the data are going to be, but it's the real data that will eventually drive your design. This goes for statistics too. Real data leads to real analysis.
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What rules of thumb do you follow?
Here's the talk in full. It's pretty interesting, if you've got about 25 minutes to spare.