  The first passage probability (FPP), of trafficked intracellular particles
reaching a displacement L, in a given time t or inverse velocity S = t/L, can
be calculated robustly from measured particle tracks, and gives a measure of
particle movement in which different types of motion, e.g. diffusion, ballistic
motion, and transient run-rest motion, can readily be distinguished in a single
graph, and compared with mathematical models. The FPP is attractive in that it
offers a means of reducing the data in the measured tracks, without making
assumptions about the mechanism of motion: for example, it does not employ
smoothing, segementation or arbitrary thresholds to discriminate between
different types of motion in a particle track. Taking experimental data from
tracked endocytic vesicles, and calculating the FPP, we see how three molecular
treatments affect the trafficking. We show the FPP can quantify complicated
movement which is neither completely random nor completely deterministic,
making it highly applicable to trafficked particles in cell biology.
