For attribute-value data, the cases to be analyzed are described by a number of attributes. The attributes to be used are specified in a names file. Each case in the data file must have a value for each attribute. The special attribute value `?’ indicates that no value is available for the attribute with respect to the current case.
Magnum Opus distinguishes between two types of attributes.
| Categorical attributes | The values for the attribute are used directly without any preprocessing. These values may include numbers, but the system treats each number simply as an identifier. |
| Numeric attributes | The values for the attribute are numeric. These values must be divided into sub-ranges. These sub-ranges are specified in the names file, or may be automatically selected by the system. |