It depends on the communication method and choice of media:
If we are trying to solve a problem by requesting feedback through surveys or using the Delphi technique, then I would consider this to be "formal written".
If we are doing the same thing just by sending emails, chats or text messages back and forth, then it's "informal written".
If we are inviting everyone to a meeting with a clearly defined agenda and timetable in order to determine a root cause, then it would be "formal verbal"
But if we happen to come across an expert in our company's coffee corner and strike up a root cause conversation with her, then that is clearly "informal verbal".
So you see that the "How" is important. You cannot just simply say "solving a complex problem = formal written" without knowing all the aspects of the communication.
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