Legal Definitions - rule of the floating subtrahend

The rule of the floating subtrahend is a legal principle that applies when a person (the plaintiff) seeks compensation for harm or damages, but the total harm they experienced was not caused entirely by the party they are suing (the defendant).

Under this rule, the plaintiff has a specific responsibility: they must clearly identify and prove the exact amount of damage that was not caused by the defendant. This portion of damage, which is essentially "subtracted" from the total harm, is referred to as the "subtrahend." If the plaintiff fails to adequately distinguish the harm caused by others or by pre-existing conditions from the harm caused by the defendant, they may be unable to recover any compensation at all, even for the damage the defendant demonstrably caused. This is because the legal system places the burden on the plaintiff to prove precisely what the defendant is responsible for.

Here are some examples to illustrate this concept:

• Example 1: Property Damage Claim

  Imagine a homeowner (the plaintiff) sues a plumbing company (the defendant) for extensive water damage to their kitchen. The homeowner claims the damage resulted from a faulty pipe installation by the company. However, during the investigation, it's discovered that a significant portion of the water damage, including some mold growth and structural weakening, was already present due to a slow, undetected leak from an unrelated appliance that existed before the plumbing company's work. Under the rule of the floating subtrahend, the homeowner must provide clear evidence to distinguish the damage caused by the plumbing company's faulty installation from the pre-existing damage caused by the appliance leak. If they cannot precisely show which damages are attributable to the plumbing company versus the prior leak, they risk recovering nothing for the kitchen damage.

• Example 2: Personal Injury Lawsuit

  Consider a person (the plaintiff) who suffers a severe knee injury in a slip-and-fall accident at a grocery store (the defendant). The plaintiff sues the store for negligence, claiming the wet floor caused their injury. However, medical records reveal that the plaintiff had a pre-existing degenerative knee condition that occasionally caused pain and limited mobility, although it was not as severe as the current injury. To recover damages, the plaintiff must present medical testimony and evidence to differentiate the pain, medical expenses, and loss of function directly resulting from the slip-and-fall accident (e.g., a new tear or significant aggravation of the existing condition) from the symptoms and limitations they would have experienced solely due to their pre-existing knee condition. If they cannot clearly separate the new harm from the old, they may not be compensated for any of their knee-related issues.

• Example 3: Business Interruption Claim

  A small manufacturing business (the plaintiff) sues a utility company (the defendant) for significant financial losses due to a prolonged power outage caused by the utility company's negligence. The business claims lost profits and operational expenses during the outage. However, the business was already experiencing a downturn in sales and production efficiency due to a recent economic recession and supply chain issues that began before the power outage. According to the rule of the floating subtrahend, the manufacturing business must provide detailed financial records and expert analysis to specifically quantify the financial losses directly attributable to the power outage (e.g., lost production during the outage, spoilage of materials) versus the losses that would have occurred anyway due to the recession and supply chain problems. If they cannot clearly isolate the impact of the power outage from these other contributing factors, their claim for lost profits might be denied entirely.