An assessment of aerosol-cloud interactions (ACI) from ground-based remote sensing under coastal stratiform clouds is presented. The assessment utilizes a long-term, high temporal resolution dataset from the Atmospheric Radiation Measurement (ARM) Program deployment at Pt. Reyes, CA in 2005 to provide statistically robust measures of ACI and to characterize the variability of the measures based on variability in environmental conditions and observational approaches. The average ACIN ( = dlnNd/dlnα, the change in cloud drop number concentration with aerosol concentration) is 0.48, within a physically plausible range of 0-1.0. Values vary between 0.18 and 0.69 with dependence on (1) the assumption of constant cloud liquid water path (LWP), (2) the relative value of cloud LWP, (3) methods for retrieving Nd , (4) aerosol size distribution, (5) updraft velocity, and (6) the scale and resolution of observations. The local radiative forcing for this range of ACIN values, assuming an aerosol perturbation of 500 cm-3 relative to a background concentration of 100 cm-3, ranges between - 4 and - 9 W m-2. Further characterization of ACI and its variability is required to reduce uncertainties in global radiative forcing estimates.