No absolute guidelines exist for determining when a report must be formal and when it may be less formal, but the following information may be used as a general guide.
Longer reports are more formal than shorter ones.
Special reports are more formal than periodic ones. Reports sent to a group of people are more formal than reports sent to one person. Reports sent to people higher in the organization are more formal than ones sent to people at the same level. Reports that go outside the work area or department are more formal than reports that stay inside the work area. Passive voice is used more often in formal reports than it is in informal ones.