1. A method for estimating combined system workloads, said method comprising: obtaining a set of quantile-based workload data pertaining to a plurality of systems; normalizing said quantile-based workload data to generate normalized workload data that compensates for relative measures between data pertaining to different ones of said plurality of systems; determining a confidence level that has been specified to control a degree of conservatism in estimating combined system workloads for said plurality of systems, the specified confidence level being selectable from a plurality of confidence levels each indicative of a proportion of scenarios to be excluded from an analysis in order to vary the degree of conservatism; using said specified confidence level to determine a degree of workload contention between said plurality of systems according to at least one probabilistic model; using said degree of workload contention and said normalized workload data to combine workloads for said plurality of systems; and storing a result indicative of one or more combined workloads according to the specified confidence level.