Results from the NPTS
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- The National Personal Transportation Survey (NPTS) for 1995 has
information on usage of vehicles and enough information to disaggregate
by age, model year, and make. One can see
the relationship between age and annual
miles. The change in odometer readings is suspect especially because
of its volatility and its negative values in some years. The median curve
implies a certain amount of bunching around round numbers (multiples of
1000). However, both the mean and the median curve show a strong downward
trend in miles per year as a function of age. The small deviation between
the two curves suggests that any outlier problem in the mean curve is
- If we disaggregate by brand,
there is significant variation in curves across brands. All of the
curves are downward sloping except for those with trancated panels.
Thus there is promise to use some brand specific mileage/aging parameters
as measures of age effects on benefits.
- Vehicle ownership per household varies with family size but in
somewhat surprising ways. Vehicle
ownership graphs show that a) in households with at least one driver,
the modal number of vehicles is 2 and b) in small households, a
significant proportion of households own more vehicles than have
drivers. The distribution of miles
by drivers in the household increases from households with one driver to
households with three but then is pretty stable. Also, the density of
miles per vehicle, ranked by vehicle, disaggregated by number of vehicles
in the household, shows that household mileage is shared among the first
two vehicles and, to a lesser extent among the third vehicle. But other
vehicles are used only marginally.
- We can model the effect of car
characteristics on mileage and estimate the parameters of the model.
Maintained by Steven Stern