What factor do Shandy and Moe point to in "Negotiating Family and Work Time in America" as the most important in understanding why women with unprecedented access to education, jobs and income "opt out" of the workforce?

Chen's research team randomly assigned some newborns and their mothers either to a breastfeeding promotion group (where breastfeeding was encouraged, but not required) or to a normal pediatric care group (where breastfeeding was neither encouraged nor discouraged). which research method did they use?