= Solution
The stop records estimate
$$
\mathbb E[Y\mid D=d,M=1]
\quad\text{and}\quad
\mathbb P(D=d\mid M=1),
$$
but they cannot estimate the population encounter probabilities $\mathbb P(D=1)$ and $\mathbb P(D=0)$ because encounters without a stop are absent. Equivalently, they do not determine the race-specific stop-probability ratio.
The scientist needs a representative denominator for all police-civilian encounters, including those with $M=0$. Suitable sources could include a carefully designed population or travel survey, systematic street and traffic observation, dispatch or body-camera sampling that records non-stop encounters, or an external administrative source measuring exposure to police by race. Combining its estimate of the population race odds with the estimable stop-data terms identifies the displayed causal risk ratio, provided the external sample targets the same city, period, and encounter population.
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