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tokens: in Kharchenko’s catalog and count the summed flux of member stars inside-out until the radius where the flux is half of the total. We also use the bootstrap realizations to estimate the error in r h . Here, not all the member stars but only the visible ones are used for the calculation of r h . The mass segregation effect (Pang et al. 2013 ) might bias the r h we measure. To test this effect, we also count the half-number radius, the radius where the number of most probable member stars is half the total, and find that it is almost indistinguishable from the r h of each OC. The error of r h is quite small at the order of 0.01–0.05 pc. 3.2. The Line-of-sight Velocity Dispersion The accuracy of RVs allows us to compute the line-of-sight velocity dispersion ( σ 1D ) among member stars. We use stars having high S/N and RV errors less than 0.125 km s −1 . Typically, these stars of high S/N also have [Fe/H] measurements. To further reduce possible biases from membership determination, we fit the [Fe/H] and RV distribution with a two-dimensional Gaussian mixture model Φ tot , which is constituted of two components, the cluster Φ c and the field Φ f . Under this assumption, the likelihood of a star with given parameters is , where n c is the fraction of the cluster component; is Gaussian distribution function; , , and are the dispersions of ; RV c (RV f ) is the mean RV of cluster (field); and M c ( M f ) is the mean metallicity of cluster (field), respectively. Note that and are not constants, but are composed of two terms, the intrinsic dispersion () and the individual observational uncertainty (), which varies by one star: Parameters are fitted so that the total likelihood of all stars reaches maximum. We use nested sampling to derive the probability density function (PDF) of parameters. Since the resulting PDF is almost Gaussian, we take the mean and standard deviation of the PDF, as the best estimation of the parameters and their errors. As an example in Figure 2 , the blue dots show the [Fe/H] and RV values for individual selected stars in NGC 1245, where the two-dimensional Gaussian mixture model (Equation ( 1 )) is projected into one-dimension to show how well the model fits the data. The [Fe/H]–RV distribution is fitted well by the sum of field (green lines) and member (red lines) components. Figure 2. Distribution of [Fe/H] and RV (blue dots) for the open cluster NGC 1245 is fitted with a two-dimensional Gaussian model. For illustration, we project this model into one-dimension. The distributions of [Fe/H] (right histogram) and RV (top histogram) are constituted of two components, the field (green lines) and the cluster (red lines). The histogram is normalized such that the integral over the range is 1. The members of OCs observed in APOGEE-2 are spatially random sampling of the OC members. Therefore, we do not expect that this random member list will bias the computation of the velocity dispersion of OCs. Typically, the RV measurements of