grouping of items and/or batches having similar physical and chemical characteristics (e.g. isotopic composition) made for the purpose of facilitating statistical  sampling.  This helps with  samplingbecause items in a particular stratum are more like one another.  This also simplifies verification, making it possible to formulate the sampling plans needed to verify a material balance and to calculate its uncertainty (σMUF).  The operator and the IAEA should co-operate in defining strata so that the purposes of stratification are met.