Set Expansion is expanding a partial set into a more complete set. Set Reduction is reducing the set to a less complete set.
I heard Josh Kopelman talk about Set Expansion / Set Reduction in relation to founders picking their startup idea on The Twenty Minute VC podcast.
Don’t set an arbitrary time. When you set an arbitrary time, you say alright, I’m going to spend this quarter picking ideas. What tends to happen is you focus on set reduction. You say “I’m going to quickly build a set of ideas and then I’m going to spend the next three months reducing them to find the one idea.” In most cases, the best pickers are people who are consistently focused on set expansion. They don’t feel the time pressure of an arbitrary deadline so they’re consistently reevaluating and consistently collecting data. — Josh Kopelman
The takeaway from Set Expansion / Set Reduction is to increase your set of ideas or options by constantly gathering information.