A computer-implemented method guides a user in designing a product defined by data. The method involves examining (420) items of the data to detect whether each item conforms to rules (106) or violates the rules; performing (431 . . . ) at least one action in response to a detection of a rule violation; displaying (450) the data to the user in a manner emphasizing at least one particular data item that caused the rule violation; receiving (460) any new data input from the user; and repeating the examining, action performing, and displaying steps using the new data. The displaying step may involve displaying data defining a current state of the product in a tabular (spreadsheet) format, including displaying data detected to have caused the rule violation in a first manner that is visibly distinct from displaying other data that is not detected to have caused the rule violation.