Box 1.

The weak ergodic assumption:

If the distribution of cells among different states does not change over time, then the proportion observed in any state is proportional to the time each cell spends, on average, in that state.

Strong ergodic assumption:

If all cells are going through an identical cycle of events, then the proportion of cells in any cycle stage observed in the population at a single time point is the same as the proportion of time spent in cycle stage as a single cell progresses through the cycle.