One way to overcome groupthink is to divide a group into smaller groups to reevaluate a plan. Why would this help to dispel groupthink?

Even if groupthink prevails in the smaller groups, the groups may at least reach different conclusions.

Groupthink occurs in large but not small groups.

Groupthink in smaller groups will tend to produce better proposals.

There is no other way to overcome groupthink than by dissolving a group and approaching a problem individually.