The present invention extends to methods, systems, and computing system program products for decrementally calculating Z-score for Big Data or streamed data. Embodiments of the invention include decrementally calculating one or more components of a Z-score for a modified computation subset based on one or more components of a Z-score calculated for a pre-modified computation subset and then calculating a Z-score for a selected data element in the modified computation subset based on one or more of the decrementally calculated components. Decrementally calculating Z-score avoids visiting all data elements in the modified computation subset and performing redundant computations thereby increasing calculation efficiency, saving computing resources and reducing computing system's power consumption.