With a data analysis plan, you know what you’re going to do when you actually sit down to do the analysis of the data you’ve gathered. It’s a vitally important thing for you to have, as it will guide how you’re going to collect your data.
After all, it’s very difficult to add in new variables afterward. For that reason, you want to make sure you’ve created your plan beforehand so that you can be sure that you’re asking all the questions you need to and you know what you’re going to do. Sure, as they say, a plan only lasts until the first shot is fired.