Component complementarity in the simple view of reading ($\partial R/\partial D=C$)

In the idealised model $R=DC$, changing decoding by $h$ with comprehension fixed changes the product by $hC$, since $(D+h)C-DC=hC$. Similarly, changing comprehension by $k$ changes it by $kD$. ThusIf either component is zero, increasing only the other leaves the product zero. Equal products can arise from different profiles: $0.3\times 0.9=0.9\times 0.3=0.27$. These algebraic properties explain the model's complementarity; they are not estimates of treatment response or diagnostic cutoffs.