Final answer:
Job descriptions are the most useful documents for grouping jobs with similar duties when an organization is updating its compensation system because they provide detailed information about the duties and responsibilities of a position.
Step-by-step explanation:
When an organization is updating its compensation system and evaluating jobs by grouping those with similar duties, job descriptions would be the most useful documents. Job descriptions provide detailed information about the duties, responsibilities, and qualifications of a given position. This specificity makes it easier to identify and group similar jobs for compensation analysis.
While questionnaires can be useful for assessing job satisfaction, and performance objectives can help measure job performance, neither provides the structured detail about job duties that a job description does. Job specifications may also be helpful as they detail the skills and qualifications required, but again, for the purpose of grouping similar positions, the broad informational scope of a job description is most relevant.