Workers’ Compensation · Primary-source case analysis

Escobedo: Apportionment Requires Reasoned Substantial Medical Evidence

By Aneeta Mishra and Timothy Chan

Published April 14, 2026

Escobedo remains a foundational explanation of post-SB 899 apportionment. It addresses both what may count as an “other factor” and what makes a medical allocation legally reliable.

Causation of disability

The Board distinguished causation of the industrial injury from causation of permanent disability. Section 4663 asks what approximate percentage of the permanent disability was directly caused by the industrial injury and what percentage was caused by other factors.

Factors beyond prior awards

Other factors can include pathology, asymptomatic prior conditions, and prophylactic work restrictions that would not necessarily have supported apportionment under earlier law. The key is whether substantial medical evidence establishes that the factor actually caused permanent disability.

What substantial evidence requires

A physician must base the allocation on reasonable medical probability, pertinent facts, an adequate examination and history, and an explanation connecting those facts to the percentages selected. A bare conclusion or speculative percentage is not enough.

Burdens and adjudication

The applicant bears the burden of establishing disability directly caused by the industrial injury, while the defendant bears the burden of proving disability caused by other factors. The judge must assess whether the medical reasoning satisfies the substantial-evidence standard.

Key takeaways

• Apportionment concerns causation of permanent disability, not simply causation of injury.
• An “other factor” must actually contribute to disability.
• Approximate percentages require a reasoned medical explanation.
• The WCAB must reject speculative or conclusory allocations.