**2.2 Statistical analysis**

The group means difference among the four hierarchical mobility disability groups was determined by analysis of variance (ANOVA) for the continuous variables and chi-square test for discrete variables. Median age difference among the four groups were investigated by Brown-Mood test. The group comparison on the depression score among the four groups was determined by using the ANOVA on the log transformed CESD score. Cox proportional hazard regression analysis was used to determine if the hierarchical mobility disability stage was a significant predictor of future IADL disability four years later (in 2003) and eight years later (in 2007) and to report its hazard ratio.

The median age onset of IADL disability of each hierarchical mobility disability group across eight years of follow up was determined by survival analysis. The survival time for 50% of participants in each hierarchical status of mobility disability to develop IADL disability (the median survival time) was determined for the whole group and separately for men and women.
