= Component complementarity in the simple view of reading
{title2=$\partial R/\partial D=C$}
In the idealised model $R=DC$, changing <Word decoding>[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 $kD$. Thus
$$
\frac{\partial R}{\partial D}=C,\qquad \frac{\partial R}{\partial C}=D.
$$
If either component is zero, increasing only the other leaves the product zero. Equal products can arise from different profiles: $0.3\times0.9=0.9\times0.3=0.27$. These algebraic properties explain the model's complementarity; they are not estimates of treatment response or diagnostic cutoffs.
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