Final answer:
An observational design is used in a time series study involving a single group with a single continuous treatment, where characteristics are observed without manipulation.
Step-by-step explanation:
The type of design that involves a single group with a single continuous treatment in a time series study is an observational design. Here, the focus is on observing and measuring specific characteristics without affecting them in any way. This contrasts with an experimental design, which would involve manipulating a variable to assess its impact on another variable. In a time, series study with an observational design, the same group is followed over time to see the effects of certain factors, but without the introduction of experimental manipulation that characterizes controlled or quasi-experimental designs.