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Time to Replace Statistics

 
The Time to Replace metrics, which appear in the Isolation Cost and Time Statistics section of the Fault Isolation Report, describe the time associated with replacing all items (using Block Replacement) for fault groups that can be isolated using the diagnostic sequence in the current diagnostic study. In calculating these metrics, eXpress converts all object replacement times to minutes. eXpress provides six Time to Replace metrics:
Minimum
The Minimum Time to Replace is the smallest aggregate object replacement time associated with replacing all items in one of the fault groups isolated by 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 Time to Replace is the largest aggregate object replacement time associated with replacing all items in one of the fault groups isolated by 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 Time to Replace is calculated by summing the aggregate replacement time 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
TTRepl(FGi)
=
the calculated Time to Replace for FGi
Expected
The Expected Time to Replace estimates the Mean Time to Replace that would be achieved if the the current diagnostic sequence were to be employed with a fielded system. The metric is calculated by multiplying each fault group's aggregate replacement time 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
TTRepl(FGi)
=
the calculated Time to Replace for FGi
Differential
The Time to Replace Differential, which is calculated by subtracting the Average from the Expected Test limit, represents how far the expected replacement time 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 require a lot of time to replace (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 excessive replacement times. This Test limit can be calculated as follows: 
where
ExpTTRepl
=
the Expected Time to Replace (defined above)
AvgTTRepl
=
the Average Time to Replace (defined above)
Also included is the Time to Replace Differential Percentage, which is calculated by dividing the Time to Replace Differential by the Average Time to Replace: 
where
ExpTTRepl
=
the Expected Time to Replace (defined above)
AvgTTRepl
=
the Average Time to Replace (defined above)
per Operating Hour
The Time to Replace per Operating Hour is calculated by dividing the Expected Time to Replace by the calculated system Mean Time Between Failures (MTBF): 
where
ExpTTRepl
=
the Expected Time to Replace (defined above)
MTBF
=
the system Mean Time Between Failures