Incorporating variables from the new pull

[andkov]

See context : https://github.com/IALSA/MAP/issues/21

In MAP documentation:

The following variables have identified as relevant for this project:

SES

• Income - Age 40: Income level at age 40 (10-level)

  Cognitive Activity

• Cognitive activity - late life: Frequency of participation in cognitively stimulating activity in late life

  Social engagement

• Social activity - late life: Frequency of participation in social activity in late life
• Loneliness: Perceived social isolation (loneliness)
• Social network size: Size of social network based on the number of children, family, and friends seen at least once a month