  We present a global study of low mass, young stellar object (YSO) surface
densities (Sigma) in nearby (< 500 pc) star forming regions based on a
comprehensive collection of Spitzer Space Telescope surveys. We show that the
distribution of YSO surface densities is a smooth distribution, being
adequately described by a lognormal function from a few to 10^3 YSOs per pc^2,
with a peak at ~22 stars pc^-2. The observed lognormal Sigma is consistent with
predictions of hierarchically structured star-formation at scales below 10 pc,
arising from the molecular cloud structures. We do not find evidence for
multiple discrete modes of star-formation (e.g. clustered and distributed).
Comparing the observed Sigma distribution to previous Sigma threshold
definitions of clusters show that they are arbitrary. We find that only a low
fraction (< 26$) of stars are formed in dense environments where their
formation/evolution (along with their circumstellar disks and/or planets) may
be affected by the close proximity of their low-mass neighbours.
