If the key variables have not been defined or are defined in a manner inconsistent with the researcher's definition, then the usefulness of the data is limited. This statement refers to the criteria for evaluating secondary data.

A. NATURE

Explanation:

The nature of the data should be examined with special attention to the definition of key variables, the units of measurements, categories used and the relationships examined. If for instance, key variables have not been defined or are defined in a manner inconsistent with the researcher’s definition then the usefulness of the data is limited.

For a content or nature of data to be used, the key variables, categories used, unit of measurements and so on must be necessary to the researcher requirements.