Working with Empty Cells in Expressions

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At any given time, some of the cells of an analytic workspace data object may be empty. An empty cell occurs when a specific data value has not been assigned to it or when a data value cannot be calculated for the cell. The value of any empty cell in an object is NA. An NA value has no specific data type. Certain functions (for example, the aggregation functions) return an NA values when the information that is requested with the function is not available or cannot be calculated. Similarly, an expression whose value cannot be calculated has NA as its value.