We use observational studies to investigate questions and provide evidence about [ Select ) and experiments to provide evidence for [ Select ] v Observational studies can also be thought of as answering questions about [ Select ] To establish cause and-effect relationships, we want to make sure the [ Select | variable is the only thing that impacts the [Select ] variable. A well designed study will take steps like random assignment to treatment groups, placebos. and blind conditions to try to eliminate [Select ] . If we are seeking to see if the Moderna vaccine is effective at preventing COVID-19 infections, we should choose [ Select | . If we are looking to see if people are wearing masks in public, we should choose [ Select ]