When researchers are studying the effect of new drug treatments on patients, bias can be introduced by patients if they are aware of who receives the placebo. Which type of research design eliminates this type of bias?

A prospective cohort study follows a group of individuals over time to observe outcomes, without controlling for whether participants know their treatment status. This design can introduce bias as participants may alter their behavior or report outcomes differently based on their knowledge of receiving a treatment or placebo.

In an observational study, researchers observe and record outcomes without intervening or assigning treatments. While this approach can provide valuable data, it does not control for biases related to participants' knowledge of their treatment status, as they are not blinded to their assigned groups.

A time series study involves repeated observations of the same variables over time, often used to assess trends. This design does not address the issue of treatment assignment awareness among participants, leaving it susceptible to bias similar to that found in observational and cohort studies.