The graphic on the page linked below is unfair to Ayn Rand, but she wouldn't care. Authoring was only one of many tools to advance her ideas. Still, the concept applies to most stories. If you don't plan your story before writing, it had better be short, or you will likely find yourself veering into the weeds, trying to keep the story alive.

http://macroexposure.com/2014/02/18/atlas-shrugged-in-one-chart/

Edited to remove the image of the chart. I tried and failed to obtain permission from the author to post the image. Not sure I would be able to choose no entry image, I substituted a photo of mine. It has nothing to do with my topic, but it's a favorite.