Ikkeadditive

Ikkeadditive is a term used in Danish, Norwegian and other Scandinavian languages to describe relationships or effects that do not follow a simple additive rule. In practical use, it denotes that the combined impact of two or more factors differs from the sum of their individual impacts. Different spellings exist, such as ikke-additiv with a hyphen in some contexts.

In statistics and econometrics, an additive model assumes that each factor contributes independently and its effect adds up linearly: y = β0 + β1x1 + β2x2. Non-additivity occurs when the joint effect cannot be captured by a sum of separate effects, often indicated by a significant interaction term or by nonlinearity, such as y = β0 + β1x1 + β2x2 + β3x1x2, where x1x2 modifies the result.

In genetics, non-additive genetic effects include dominance and epistasis, where the genotype’s effect is not a simple sum of alleles. In ecology, nutrient or stress interactions among species or factors can be non-additive, altering outcomes beyond additive predictions. In risk assessment and pharmacology, non-additive effects describe interactions among stressors or drugs that change the overall risk or efficacy beyond a linear combination.

Detecting non-additivity is important for accurate inference. Analysts may include interaction terms, use nonlinear models, or apply non-additive risk frameworks. Recognizing non-additivity helps explain why predictions based on additive assumptions can misestimate real-world outcomes.