The method of sampling the selected variable. An 'absoluteValue' is simply the present value of the object. A 'deltaValue' is the present value minus the previous value, which was sampled expExpressionDeltaInterval seconds ago. This is intended primarily for use with SNMP counters, which are meaningless as an 'absoluteValue', but may be used with any integer-based value. When an expression contains both delta and absolute values the absolute values are obtained at the end of the delta period.