A letter to the Straits Times Forum.
The Letter
I refer to the letter, “Don’t underplay maintenance at the cost of severe rail disruptions” (June 30).
The writer insinuates that SMRT did not “consider the time cost of severe delays inflicted on commuters through disrupted commutes, as well as the accompanying psychological stress.” He did not fully understand the significance and implications of the mean kilometres between failures (MKBF) metric.
MKBF does not mean anything by itself. It’s the concomitant problems it generates such as the ones the writer mentioned, or broader ones like the harm to economy-wide productivity and public transit reputation, that matter. The costs of delay and stress are implicit in all MKBF values, because MKBF would be meaningless otherwise.
The writer is concerned that “high-impact events” are not given adequate consideration when SMRT is determining the optimal level of maintenance. This is also because the MKBF value SMRT reports is only for disruptions lasting more than 5 minutes, which cannot capture how the disruptions are distributed by time. This shortcoming can be resolved by considering multiple MKBF values for disruptions of multiple time ranges, which SMRT probably does.
Moreover, the writer states that MKBF “is a statistical average that does not take into account extreme events”. Averages by definition always account for outliers, though not necessarily to a desired degree.
The reported MKBF does not account for protracted train disruptions because SMRT discloses a sweeping measure for all disruptions lasting more than 5 minutes, not multiple MKBF values graduated by time. It’s a problem of the definition of the metric, not a problem of the mathematical concept of average.
Comments
My submission did not get published. One that replied to the same letter did. It held a similar stand as me, criticising many of the author’s points.
After taking many trips on the public transits in Hong Kong, Taipei and Tokyo, I feel that public transport in Singapore is great. It has improved massively in quality and reliability in the past 10 years, and the public transport network is still continuing to grow.
Extension
Nicely designed information sheets by SMRT.
BLOGOPUS 
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