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Time-Series Study

Time-series analysis (TSA) is a statistical methodology appropriate for longitudinal research designs that involve single subjects or research units that are measured repeatedly at regular intervals over time. TSA can be viewed as the exemplar of all longitudinal designs. TSA can provide an understanding of the underlying naturalistic process and the pattern of change over time, or it can evaluate the effects of either a planned or unplanned intervention. The advances in information systems technology are making time-series designs an increasingly feasible method for studying important psychological phenomena.

Modern TSA and related research methods represent a sophisticated leap forward in the ability to analyze longitudinal data. Early time-series designs, especially within psychology, relied heavily on graphical analysis to describe and interpret the results. Although graphical methods are useful and provide important ancillary information, the ability to bring a sophisticated statistical methodology to bear has revolutionized the area of single-subject research.

TSA was developed more extensively in areas such as engineering and economics before it came into widespread use within social science research. The prevalent methodology that has developed and been adapted in psychology is the class of models known as Autoregressive Integrated Moving Average (ARIMA) models. TSA requires the use of high-speed computers; the estimation of the basic parameters cannot be performed by precomputer methods.

Figure 1 Smoking Behavior Measured on 124 Occasions: An Example of Time-Series Data for a Single Individual

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A major characteristic of time-series data is the dependency that results from repeated measurements over time on a single subject or unit. All longitudinal designs must take dependency into account. Dependency precludes the use of traditional statistical tests because they assume the independence of the error. ARIMA models have proven especially useful because they provide a basic methodology to model the dependency from the data series and allow valid statistical tests. This entry discusses several aspects of TSA, including its application in research, modeling procedures, weakness and confounders, and the use of telemetrics in TSA.

Research Applications

As the methodology for TSA has evolved, there has also been increasing interest among applied researchers. Many behavioral interventions occur in applied settings such as businesses, schools, clinics, and hospitals. More traditional between-subject research designs might not always be the most appropriate, or in some instances, these designs can be very difficult to implement in such settings. In some cases, data appropriate for TSA are generated on a regular basis in the applied setting, like the number of hospital admissions. In other cases, a complete understanding of the process that can explain the acquisition or cessation of an important behavior might require the intensive study of an individual during an extended period of time. Advances in information systems technology have facilitated the repeated assessment of individuals in natural settings. In addition, there has been increasing concern about the appropriateness of group methods based on the ergodic theorems. The ergodic theorems state that an analysis of the interindividual variation yields the same results as an analysis of intra-individual variation only if the trajectory of each subject obeys the same dynamic laws (i.e., the same autocorrelation structure), and each individual trajectory has the same statistical characteristics (i.e., equal mean level and temporal pattern).

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