Aresearcher is interested in determining if infant crying and caretaker responses are different in developed and developing nations. she trains her research assistants to watch and record babies' behaviors in 3-minute increments, including variations in crying (e. g., whimpering, pain cries, as well as caregiver responses (e. g., holding, speaking, she then chooses 50 participants in four different countries to be included in her study. what type of research design is she using?