fluids are less common, though note, e. g., Streletzky and Phillies[@streletzky1998q]. These authors found spectra having multiple relaxational modes, some of which had relaxation rates that did not scale linearly in $q^{2}$, proving that the modes did not correspond to Gaussian displacement distribution functions.

Particle tracking is sometimes used to generate the simplified $\langle \Delta x(t)^{2} \rangle$, rather than the full $P(\Delta x, t)$. The simplification is potentially hazardous, because if one has not determined $P(\Delta x, t)$ one does not know if the particle motion process corresponds to simple diffusion. Any physically reasonable $P(\Delta x, t)$ has some second moment $\langle \Delta x(t)^{2} \rangle$, but if the form of $P(\Delta x, t)$ is unknown, one cannot tell if it is meaningful to characterize $P(\Delta x, t)$ by its second moment or by the corresponding nominal diffusion coefficient $$D(t) = \langle \Delta x(t)^{2} \rangle/ 2 t
      \label{eq:timedependentD}$$ The notion that a diffusive process can be characterized by $\langle \Delta x(t)^{2} \rangle$ corresponds to particle diffusion in simple Newtonian liquids, in which $D(t)$ as defined here is a constant independent of time. In a complex fluid, characterization of probe motion via measurement of the second moment $\langle \Delta x(t)^{2} \rangle$ must 