Each 0 represents an observation at a different point in time. These designs do not require a control or comparison group (though clearly the addition of one would enhance the design). In the notation above there are five pre-intervention observations, followed by the intervention represented by X, which in turn is followed by five more observations. In our psychiatric patient example, instead of having a comparison group, a time series design may involve measuring symptom severity each week for 5 weeks before the intervention period and 5 weeks following. Examination and statistical comparison of the pre- and post-intervention slope can provide indications of the potential effect of the intervention.