Findings for person parameters

The Rasch model also helped us to estimate the self-rated vehicle dependence of the 321 motorcyclists who participated in the study. Some summarized relative measures of vehicle dependence and fit statistics for persons are shown in Table 4-4. The raw score of each person has been transformed monotonically into a measure of motorcycle dependence on a logit scale from −2.96 to 5.00. The fit statistic for each person’s motorcycle dependence estimate is generally acceptable, which shows that the person responses generally fit the Guttman scale (Guttman, 1950) well.

Figure 4-2 illustrates the distribution of infit and outfit Zstd statistics for the estimates of θ . Each data point represents the infit and outfit Zstd values for an individual respondent.

About 15% of the surveyed motorcyclists have fit statistics outside the ± 2 Zstd tolerance box. The scattered points located in the upper right area outside the tolerance box imply that these person responses are over-dispersed compared with the Guttman scale (Smith, 1992).

Such a result is generally derived from a violation of the monotonic property in the Rasch model. That is, respondents might achieve the requirement easily for items with higher

difficulty but behave poorly for items with lower difficulty. Oppositely, the scattered points located in the lower left area outside the tolerance box in Fig. 4-2 imply that these person responses are under-dispersed compared with the Guttman scale. Such results might arise from respondents who are unable to judge the relative level of categories well; that is, their responses do not vary with the difficulty of items. Both over-dispersed and under-dispersed responses are regarded as indicating respondents that may possibly violate the properties of the Rasch model. If a person estimate is going to serve as a variable in an extended study, it has been suggested that these questionable responses should be modified or deleted (Bond and Fox, 2001).

Table 4-4 Summarized estimates of person parameters and fit statistics from Rasch analysis Person

In order to explore the useful information contained in the person parameters estimated from the Rasch model, the estimated person parameters were then further investigated by

studying their corresponding personal characteristics. Only the 273 well-fitted respondents, who obeyed the assumptions of the Rasch model, were used for this extended study and discussion. The self-rated motorcycle dependence and personal characteristics of these 273 well-fitted motorcyclists are shown in Table 4-5.

-4 -3 -2 -1 0 1 2 3 4 5

-4 -3 -2 -1 0 1 2 3 4 5 6

Outfit Zstd Infit Zstd

Fig. 4-2. Scatter plot of infit and outfit statistics for estimates of person measures. Each point represents a different respondent. (The square bounds the 95% confidence limits) These respondents were categorized into two groups for each characteristic, namely age, gender, monthly income, and automobile-driving status. The 144 respondent motorcyclists who also traveled by automobile are referred to here as dual-mode users. These dual-mode users were further divided into two groups by their ownership of a personal, exclusive parking space. The average measures of motorcycle dependence for each group were calculated and compared with their counterpart groups. The statistical significance of the mean differences between groups for each social characteristic was tested, and the p-value is shown in the last column in Table 4-5.

The average motorcycle dependence for motorcyclists aged under 25 is significantly

higher than that for motorcyclists aged over 25. This implies that younger motorcyclists in Taipei tend to depend more on motorcycle riding to achieve their trips than older motorcyclists do. The difference between the motorcycle dependence of male and female motorcyclists is not significant, which indicates there is no obvious gender gap in the motorcycle dependence of motorcyclists in Taipei. Motorcyclists with a monthly income lower than 30,000 NTD depend on motorcycles significantly more than do those with a monthly income higher than 30,000 NTD. In view of the average monthly income per capita of 40,142 NTD in Taipei (Official Statistics, 2005), it appears that those who are economically inferior in Taipei tend to depend more on motorcycles to travel.

Table 4-5 Self-rated motorcycle dependence for different groups of motorcyclists Personal characteristics Group Number of

observations

** This analysis was based on the 144 dual-mode users.

The dual-mode users have significantly lower dependence on motorcycles than have those

motorcyclists who do not drive automobiles. It might be the case that the former have the

“car-driving” alternative for travel available, and therefore depend less on motorcycle riding than do the latter. This is consistent with the findings of previous studies (Chang, 2002) that automobile driving and motorcycle riding are partially competitive for residents of Taipei.

In addition, the dual-mode users who owned their personal automobile parking spaces were found to have less motorcycle dependence than those who did not own automobile parking spaces. In such an intensively occupied city as Taipei, owning a personal, exclusive automobile parking space is very costly. It is much harder to find a free or cheap automobile parking space than to find a free motorcycle parking space in Taipei. Therefore, once a dual-mode user without his/her own automobile parking space parks his/her automobile on a cheap or free parking space, that person will try his/her best to travel by motorcycle unless he/she is forced to travel by automobile to perform some special activity. This is a common behavior in Taipei, especially for those who belong to the medium-income group, and can be used to explain the relationship between automobile parking space ownership and motorcycle dependence for dual-mode users.