Table A2.

Early Developmental Milestones.

Early Development Milestones	Definition
Walking alone	Has the child started to walk by herself/himself? How old (in months) was the child when she/he started walking? Notes. Walking refers to walking on one’s won without assistance of any other objects of external forces. Walking 3–5 steps even counts. Soft check. Interviewer’s notes. Double check in case the child was able to walk after 24 months old.
First sentences	Has the child started to speak complete sentences, such as “I want to eat”? How old (in month) was the child when she/he started speaking complete sentences, such as “I want to eat”? Soft check. Interviewer’s notes. Double check in case the child was able to walk before 12 months old and after 24 months old.
Counting 10 objects	Can the child count from 1 to 10? How old (in months) was the child when she/he became able to count from 1 to 10? Soft check. Interviewer’s notes. Double check in case the child was able to walk before 12 months old and after 24 months old.
Independent toileting	When the child urinates, is she/he able to take off the trousers on her/his own? How old (in months) was the child when she/he became able to urinate by herself/himself? Notes. “Urinate by himself/herself” refers to the child can take off his/her trousers on his/her own when urinates. Soft check. Interviewer’s notes. Double check in case the child was able to walk before 12 months old and after 36 months old.

Notes. The Computer-assisted personal interviewing (CAPI) system adopted by the CFPS is capable of performing soft range checks, making it possible for interviewers to query implausible responses instantly.