This warning is based on observations of many student database designers getting the degree of relationship types wrong. The usual cause of error is only asking one question and not starting with the word 'one'. For example a student might say (incorrectly): 'Many customers receive many invoices' (which is true) and wrongly conclude that the relationship type is many-many. The second most common source of error is either to fail to name the relationship type and say something like 'Customer to Invoice is one-to-many' (which is meaningless) or give the relationship type an inappropriate name.