Total 169 tunnel segments (a segment is defined as the tunnel
tube between two stations) are inspected on the six subway lines.
Multiple defects are observed in all segments. For each defect, a
failure is quantified by the number of observed defect in a tunnel
segment between two subway stations. In an engineering sense,
it is reasonable to assume that the number of observed defects increases
over the time, provided that no maintenance activity was
taken after the corresponding defect was observed previously. In
this study, the number of defects in a tunnel segment is normalized
by the segment length, resulting in the defect ratio per kilometer,