 Cartwright requires \(\b{K}\) to include all causes of \(\r{E}\), and thus to evaluate effects relative to homogenous background contexts. The account thus does not allow for average effects. For example, suppose that a particular treatment \((\r{T})\) raises the probability of heart disease \((\r{H})\) in individuals who were born prematurely \((\r{P})\) but not individuals who were not, and that \(\r{P}\) is not correlated with \(\r{T}\). In the whole population, the amount by which \(\r{T}\) lowers or raises the probability of \(H\) will be an average of the effects in the \(\r{P}\) and \(\neg\r{P}\) populations, weighted by their size. Dupré (1984) argues for abandoning the requirement that \(\b{K}\) include all causes of \(E\), and thus for allowing average effects.