Studies in four different organizations developing different types of software show conclusively that sizes of sprints or iterations measured using COSMIC Function Points (CFPs) correlate with effort much better than do Story Points (SP’s). This means that CFP sizes should be much better for estimating Agile activities than using SPs.
The COSMIC Functional Size Measurement method is an ISO standard way of measuring a size of software functional requirements based on fundamental software engineering principle. Hence CFP sizes are independent of the development method or technology used, the developer skills, etc.  Agile User Stories can be measured in units of CFP as well as any aggregations of Stories, i.e. sprints or iterations, releases and whole systems. In contrast, SPs have any meaning only within an individual team; they are typically used as a means for estimating team effort for User Stories and sprints, but cannot be used reliably for measuring or tracking performance across teams or for estimating effort at any higher levels of aggregation.
Agile work-units may be measured using CFP instead of SP without the need to change any Agile processes at the team level e.g. ‘Planning poker’, or at higher levels.
Download the summary report ‘Experience of using COSMIC Function Points in Agile projects’ showing the study findings here.