*2.1. Participants*

Participants of phase 1 (cross-cultural adaptation) and phase 2 (testing the new measure) were different. For phase 1, nineteen participants took part, including supervisors and home visitors from the *Primeira Infância Melhor* program (*N* = 17) and two child health university professors.

The phase 2 study was based on a subsample of the 2015 Pelotas Birth Cohort Study [31] when the children were 18 months old. The target sample size was *N* = 155, based on budgetary constraints and practice in the field [32,33]. The Pelotas cohort used demographic data collected within two days of the child's birth to identify a subpopulation of children eligible for the current parenting study. Three hundred and ninety-five families satisfied the selection criteria (full-term, singleton, normal birth weight, and aged between 17 and 18 months during a six-week data collection period). Families were stratified by wealth quintiles according to household assets assessed at the child's birth and recruited until the target sample size was achieved. Recruiting stopped after 178 families were contacted, 23 families refused (13%) and the target sample of *N* = 155 was achieved. Observational data were collected for 155 mother–child dyads. Two film clips were excluded for technical reasons (duration of less than 5 min and third-party interference in the task). The final sample of 153 dyads was determined to be optimal based on the proposed analyses

and expected results [34]. Mean child age was 17.9 months (SD = 0.27), mean gestational age was 39.4 weeks (SD = 1.23; range = 37–41.9 weeks). Females outnumbered males (females = 55.5%, males = 44.5%). The participating families' socioeconomic levels were represented equally in the upper four quintiles with about 22% each, while the first quintile (the poorest) accounted for about 12% of the children. Children in the 2015 Pelotas Birth Cohort were followed up at 24 months of age, and developmental outcomes were collected [35]. The relationship between our new parental responsivity measure and children's developmental outcomes could therefore be examined in the 153 families of our sample.
