My boss would take a piece of data/input and run it through the entire process. It's a string data here, converts to number here, function transforms it here, summarized here, output format there... You wouldn't run into data type issues or have an epiphany that you're missing a data requirement.
If the data transformations are the hard part, sure. But often the hard part is whether you're even outputting the right thing at all. Also, if you're planning in that much detail, you might as well be writing code (perhaps with some holes).
