We examine the methods used to quantify the indirect effect of aerosols on clouds using satellite retrievals of cloud droplet concentration and aerosol optical depth (AOD) or aerosol index (AI). We use a model to show that methods that use the slope of the relationship between droplet concentration and AOD may underestimate the forcing by between a factor of 3 and 6, while methods that use the slope of the relationship between droplet concentration and AI may be incorrect by between 25%-35% on a global average basis. We explore the role of different particle types in determining the relationship between aerosol properties and droplet concentration. We also look at whether values of the relationship between droplet concentration and AOD or AI in the pristine southern hemisphere may be used to estimate pre-industrial droplet concentrations over oceans. We conclude with recommendations for how to proceed to improve estimates of indirect forcing.