The common information effect is best described as the tendency for groups to:

The answer is "discuss and consider information that they all have in common more than unique information (that only one person in the group may be aware of)."

One of the most serious issues in groups is the so called common information effect. And it works this way. 
Partner A goes over some data, colleague B runs over some data, and partner C runs over some data. Some of this data will be remarkable to them, yet some will be known to either of the other colleagues, that is common information.