A method for processing data generated by instrumented software (130a, 130b, 130c, 130d), the method comprising: receiving, by an analysis system information identifying a set of data streams, each data stream generated by an instance of instrumented software (130a, 130b, 130c, 130d) executing on an external system (120a, 120b, 120c); receiving a specification of an expression, the expression processing data values of the plurality of data streams, each of the data values associated with a time interval of a plurality of time intervals for evaluating the expression; and for each of a plurality of time intervals for evaluating the expression: for each data stream of the set of data streams, determining (620, 730) a maximum expected delay value for the data stream, the maximum expected delay representing a limit on arrival delay within the time interval for a data value of the data stream from a corresponding instance of an instrumented software (130a, 130b, 130c, 130d) to the analysis system, wherein the maximum expected delay is less than a length of the time interval, monitoring the set of data streams for arrival of data values for the time interval, modifying the set of data stream for the time interval by excluding a data stream if the data value of the data stream fails to arrive within the maximum expected delay for the data stream, evaluating the expression using the arrived data values of the modified set of data streams for the time interval, and sending the value of the evaluated expression for presentation. receiving, by an analysis system information identifying a set of data streams, each data stream generated by an instance of instrumented software (130a, 130b, 130c, 130d) executing on an external system (120a, 120b, 120c); receiving a specification of an expression, the expression processing data values of the plurality of data streams, each of the data values associated with a time interval of a plurality of time intervals for evaluating the expression; and for each of a plurality of time intervals for evaluating the expression: for each data stream of the set of data streams, determining (620, 730) a maximum expected delay value for the data stream, the maximum expected delay representing a limit on arrival delay within the time interval for a data value of the data stream from a corresponding instance of an instrumented software (130a, 130b, 130c, 130d) to the analysis system, wherein the maximum expected delay is less than a length of the time interval, monitoring the set of data streams for arrival of data values for the time interval, modifying the set of data stream for the time interval by excluding a data stream if the data value of the data stream fails to arrive within the maximum expected delay for the data stream, evaluating the expression using the arrived data values of the modified set of data streams for the time interval, and sending the value of the evaluated expression for presentation. for each data stream of the set of data streams, determining (620, 730) a maximum expected delay value for the data stream, the maximum expected delay representing a limit on arrival delay within the time interval for a data value of the data stream from a corresponding instance of an instrumented software (130a, 130b, 130c, 130d) to the analysis system, wherein the maximum expected delay is less than a length of the time interval, monitoring the set of data streams for arrival of data values for the time interval, modifying the set of data stream for the time interval by excluding a data stream if the data value of the data stream fails to arrive within the maximum expected delay for the data stream, evaluating the expression using the arrived data values of the modified set of data streams for the time interval, and sending the value of the evaluated expression for presentation.