According to the PMBOK® Guide, specifically within the Monitoring and Controlling Process Group, Variance Analysis is a key data analysis technique used across multiple knowledge areas (Scope, Schedule, Cost).

Cause and Degree of Difference: The primary purpose of variance analysis is to review the difference (or variance) between planned performance (the Baseline) and actual performance. It involves:

Determining the cause: Investigating why the variance occurred (e.g., resource shortages, scope creep, or underestimated durations).

Determining the degree: Quantifying how far off the project is from its baseline (e.g., $5,000 over budget or 3 days behind schedule).

Decision Making: By understanding the cause and degree, the project manager can determine if corrective or preventive actions are required to bring the project back into alignment with the management plan.

Why the other options are incorrect:

A. Product analysis: This is a tool used in the Define Scope process to translate high-level product descriptions into meaningful deliverables. It does not measure performance against a baseline.

C. Document analysis: This is a data gathering technique used in Collect Requirements or Identify Stakeholders to elicit requirements by analyzing existing documentation.

D. Decomposition: This is a technique used in Create WBS and Define Activities. It involves breaking down project scope and deliverables into smaller, more manageable components. It is a planning tool, not a performance measurement tool.