The attribute X's included those that required carriers' immediate attention as identified in the
second stage survey. After formulating the choice model, the next step of experiment design
was to specify attribute levels to be combined to describe alternative service bundles in
choice situations. To enhance the realism of the survey, selected attribute levels should cover
a range similar to that actually experienced by shippers. Both the highest possible level and
the lowest possible level should be included. Typically two and five levels should be
assigned to each attribute but the same number of levels should be included for each attribute
to avoid bias in the estimated importance (Lilien and Rangaswamy, 2003). In our study,
three levels were assigned to each attribute. Given the data provided by the shippers
participating in our survey, the plausible values for attributes to be used in constructing
service bundles were shown in Tables 3 and 4.