Apportionment is where California PD ratings are won and lost. The rating math is mechanical; whose fault the disability is — industrial injury versus everything else — is the part the parties actually fight about, and it directly scales the award. Here’s how it works and exactly where it lands in the calculation.
Two statutes, two different tools
§4663 — apportionment to causation
Since SB 899 (2004), every physician who addresses permanent disability must also address causation: what approximate percentage of the disability was caused by the industrial injury, and what percentage by other factors — a prior injury, degenerative disease, or any non-industrial condition (Lab. Code §4663). The employer is liable only for the industrial share (§4664(a)). Causation apportionment doesn’t apply to the public-safety presumption injuries listed in §4663(e) — a firefighter’s presumed heart trouble or cancer, for example. The percentages may be approximate, but the analysis may not be casual.
§4664(b) — the prior-award presumption
If the worker already received a PD award for the same body region, that prior disability is conclusively presumed to still exist (Lab. Code §4664(b)). No medical opinion is needed to prove the old disability still exists — but it subtracts only to the extent the employer proves it overlaps the new one (Kopping v. WCAB (2006) 142 Cal.App.4th 1099). The fight there is usually about overlap: whether the old and new disabilities actually cover the same ability to work.
What a valid opinion must say
An apportionment opinion has to be substantial medical evidence — it must explain how and why the non-industrial factor causes part of the permanent disability, not merely assert “50% degenerative.” The controlling framework is Escobedo v. Marshalls (2005) 70 Cal.Comp.Cases 604 (Appeals Board en banc). A percentage without reasoning is the most common way an apportionment defense falls apart — and the first thing to test in cross-examination or a supplemental-report request.
Where it lands in the math
Apportionment is applied per impairment, after the age adjustment and before the Combined Values Chart:
The rating string leads with the industrial percentage. In the shoulder string from how to read a rating string, the age-adjusted rating is 23 and the industrial share is 80%:
Only then are the post-apportionment finals combined. A case with three body parts can carry three different apportionment percentages, each defended (or attacked) on its own medical record.
The dollars fall faster than the rating
Because the §4658 weeks schedule is progressive — higher ratings earn more weeks per point — apportionment removes the most valuable points first:
The same rating at 80% industrial becomes 16% — 55.5 weeks × $290 = $16,095.
A 20% cut in the rating took 26.5% of the dollars. The deeper the apportionment, the steeper this effect.
That asymmetry is why a few percentage points of causation are worth litigating — in either direction. Run both scenarios side by side in the calculator and the spread is on screen in seconds.
Separate injuries, separate awards
When disability comes from two different industrial injuries (say, a 2019 claim and a 2023 claim), that isn’t §4663 apportionment between industrial and non-industrial — it’s Benson: each injury generally gets its own rating and its own award (Benson v. WCAB (2009) 170 Cal.App.4th 1535), unless the physician cannot parcel out causation between them. Two 15% awards are usually worth less than one combined 28% — the same progressive-weeks arithmetic as above, working against the applicant this time.