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"S" Curves
Also referred to a symmetrical curve or bell curve or normal distribution.

One of three broad classes of 31 Iowa Curves.
used to develop probability distributions for forecasting the service life of similar assets under different operating conditions.



Attributes
Listed below are some of the key attributes of symmetrical curves:
  • The modal year and the average life are the same, thus producing a symmetrical curve.
  • Half of the assets are retired prior to the average life, and an equal amount are retired after the average life term is reached.
  • There is no skewness in the curve.
  • The early start year and the late start year are equidistant from the critical year.

Evaluation
Although there are few assets that fall into this class of distribution, the vast majority of deterioration models are based upon this form of distribution.
"S" curve (in "red") with perfect symmetry compared to skewed curves.
Fig. "S" curve (in "red") with perfect symmetry compared to skewed curves.

"S" curve indicating the percentage of performers along the probability distribution.
Fig. "S" curve indicating the percentage of performers along the probability distribution.

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