Since IT systems are always working on data. You MUST define some solid data principles. These principles will help you design the system and avoid complexity.
Below are some examples.
Statement: Data is made available as quickly as necessary to preserve the value of the data.
Statement: Data is reasonably structured to allow automated processing.
Statement: Data is as collected at the source, with the highest possible level of granularity, not in aggregate or modified forms. Rationale
Statement: Data is available to the widest range of users for the widest range of purposes. Rationale