Not All Social Programs Are Amenable to Basic Comparison Group

Not all social programs are amenable to basic comparison group designs, particularly if they involve an entire population. One alternative design that assesses the effect of a quasi-experimental treatment is a time series design. Researchers used a time series design to assess the impact of a new tax on fatty foods. They concluded that prior to the tax implementation on January 1, fatty food consumption was very high; after the tax went into effect, fatty food consumption declined considerably. Data were collected every four months. Before their conclusion can be accepted, what other factors are particular concerns for time series designs that must be taken into account? (circle all that apply)

A) data observed at one time point tend to be less related to adjacent time points than other points in the series
B) data observed at one time point tend to be more related to adjacent time points than other points in the series
C) there tend to be systematic cycles of change (e.g., with the types of food people eat) that would lead consumption to rise and fall, independent of any treatment or tax implementation
D) if data are nonlinear, it makes time series analyses extremely hard to interpret