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Cost to Isolate Statistics

 
The Cost to Isolate metrics, which appear in the Isolation Cost and Time Statistics section of the Fault Isolation Report, describe the cost associated with detecting and isolating a primary failure using the diagnostic sequence in the current diagnostic study. The currency used for this metric (e.g., US Dollars) is listed below the "Cost to Isolate" header. eXpress provides six Cost to Isolate metrics:
Minimum
The Minimum Cost to Isolate is the smallest cumulative test cost involved with detecting and isolating a failure to one of the fault groups in the current diagnostic sequence. If the analyst clicks on this field when viewing the Fault Isolation Report in the eXpress report viewer, it will bring up a details report describing the fault group for which this Test limit was derived.
Maximum
The Maximum Cost to Isolate is the largest cumulative test cost involved with detecting and isolating a failure to one of the fault groups in the current diagnostic sequence. If the analyst clicks on this field when viewing the Fault Isolation Report in the eXpress report viewer, it will bring up a details report describing the fault group for which this Test limit was derived.
Average
The Average Cost to Isolate is calculated by summing the isolation cost associated with each isolated fault group and dividing the sum by the total number of isolated fault groups:
where
N
=
the total number of isolated fault groups (Total Fault Groups)
FGi
=
the ith isolated fault group
CTI(FGi)
=
the calculated Cost to Isolate for FGi
Expected (MCTI)
The Expected Cost to Isolate  estimates the Mean Cost to Isolate (MCTI) that would be achieved if the current diagnostic sequence were to be employed with a fielded system. The metric is calculated by multiplying each fault group's Cost to Isolate by the fault group's aggregate failure probability and then summing the results for all isolated fault groups:
where
N
=
the total number of isolated fault groups (Total Fault Groups)
FGi
=
the ith isolated fault group
Afp(FGi)
=
the aggregate failure probability associated with FGi
CTI(FGi)
=
the calculated Cost to Isolate for FGi
Differential
The Cost to Isolate Differential, which is calculated by subtracting the Average from the Expected Test limit, represents how far the expected isolation cost will deviate from the non-probability-weighted average. This can be a good indication of how well items or functions that fail frequently are being segregated from those that cost a lot to isolate (a high positive differential Test limit indicates that they are well segregated, whereas a negative Test limit may indicate that they are poorly segregated). This metric, which was developed by DSI in the early 1990s, provides the analyst with information that may help identify the source of exorbitant isolation costs. This Test limit can be calculated as follows:
where
ExpCTI
=
the Expected Cost to Isolate (defined above)
AvgCTI
=
the Average Cost to Isolate (defined above)
Also included is the Cost to Isolate Differential Percentage, which is calculated by dividing the Cost to Isolate Differential by the Average Cost to Isolate: 
where
ExpCTI
=
the Expected Cost to Isolate (defined above)
AvgCTI
=
the Average Cost to Isolate (defined above)
per Operating Hour
The Cost to Isolate per Operating Hour is calculated by dividing the Expected Cost to Isolate by the calculated system Mean Time Between Failures (MTBF):
where
ExpCTI
=
the Expected Cost to Isolate (defined above)
MTBF
=
the system Mean Time Between Failures