Table 1.

Variable definition and assignment

Independent variable	Variable definition and assignment
Socioeconomic status
Income	Annual household income. 0 = less than 10,000 yuan (Very low-income); 1 = 10,000-30,000 yuan (low-income);2 = 30,000-9000 yuan (medium income); 3 = 90,000 yuan and above (high income)
Education	The highest degree so far.0 = illiteracy;1 = primary;2 = junior;3 = senior and above
Occupation	Work before retirement.0 = agriculture;1 = governmental & professional;2 = commercial & service
Health status
IADL	Value range:8-24 points
MMSE	Value range:0-30 points
Mediating variables
Group exercise	Such as square dance/Tai chi etc.; Value range:3-15 points
Organized social activities	Such as volunteer etc.; Value range:1-5 points
Interact with friends	Such as play cards/mah-jongg etc.; Value range:2-10 points
Covariate
Age	0 = Aged 75 or younger; 1 = 75-84 years old; 2 = 85-94 years old; 3 = Aged 95 or older
Gender	0 = male;1 = female
Residential area	0 = city;1 = town;2 = rural
Marital status	0 = married and living with a spouse;1 = separated (widowed or divorced)
District	0 = East;1 = Central;2 = West;
Accessibility of health services	The distance of the nearest hospital from home .0 = within 1 km; 1 = 1-3 km; 2 = up to 3 km;
Medical insurance	0 = no;1 = yes;
Pension insurance	0 = no;1 = yes;