English: Arbitrary numbers are provided for observed quantities, such as the proportion of infections due to subtype 1 in source population 2 (${\displaystyle p_{12}}$). The marginal effect associated with source population 2 (${\displaystyle a_{2}}$) is represented by an open rectangle (solid line); while the total size of this source population ${\displaystyle M_{2}}$ is observed, ${\displaystyle a_{2}}$ must be estimated by regression. Similarly, the marginal effect associated with subtype 3 (${\displaystyle q_{3}}$), indicated by a rectangular shaded region) is simultaneously estimated by regression.