Evolution of spatio-kinematic structures in star-forming regions: are
  Friends of Friends worth knowing?
  The Friends of Friends algorithm identifies groups of objects with similar
spatial and kinematic properties, and has recently been used extensively to
quantify the distributions of gas and stars in young star-forming regions. We
apply the Friends of Friends algorithm to $N$-body simulations of the dynamical
evolution of subvirial (collapsing) and supervirial (expanding) star-forming
regions. We find that the algorithm picks out a wide range of groups (1 -- 25)
for statistically identical initial conditions, and cannot distinguish between
subvirial and supervirial regions in that we obtain similar mode and median
values for the number of groups it identifies. We find no correlation between
the number of groups identified initially and either the initial or subsequent
spatial and kinematic tracers of the regions' evolution, such as the amount of
spatial substructure, dynamical mass segregation, or velocity dispersion. We
therefore urge caution in using the Friends of Friends algorithm to quantify
the initial conditions of star formation.
