= Molecular population
{title2=$p_i=N_i/N$}
A <molecular population> is a specified collection of <molecules>, or the count occupying a selected molecular state. The definition includes the chemical species, compartment, time and state classification. If $N_i$ molecules occupy state $i$ out of $N$ total, its population fraction is $p_i=N_i/N$ and $\sum_i p_i=1$ for an exhaustive, nonoverlapping classification.
An experimental signal need not equal that fraction. In a simple additive <fluorescence> model,
$$
F=\sum_i b_iN_i,
$$
where $b_i$ is the detected <molecular brightness>, the fluorescence per molecule in state $i$. If two states are equally populated but one is twice as bright, it contributes two thirds of the signal, despite containing half the molecules. Equal molecular counts are therefore not inferred from equal signal contributions without a response model.
<Single-molecule fluorescence microscopy> can resolve trajectories or individual emitters, while ensemble methods average over many molecules. Detection, labeling and state-dependent response still determine which <molecular population> is represented. A <conformational population> is one important special case, where the classification records molecular shape rather than chemical identity.
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