Probability distributions of YSO surface densities in units of pc^-2 and logarithmic scale. The solid curve represents our assumed distribution for the clustering experiments (lognormal with mean μ_log 10Σ = 2 and dispersion σ_log 10Σ = 1), whereas the dashed curve indicates the distribution by Bressert et al. (2010) for several Spitzer-observed star-forming regions within 500 pc from the Sun (lognormal with mean μ_log 10Σ = 1.34 and dispersion σ_log 10Σ = 0.85). The normalized histogram is the combined surface density distribution of the 17 massive star-forming regions studied by Kuhn et al. (2015).