Série Temporal em Estudos de Saúde

PúblicaClarice Amorim e Jessica Johnson

Interrupted Time-Series

Clarice Amorim & Jessica Johnson

● Overview of Interrupted Time-Series (ITS)

○ Types of Designs○ Validity Threats○ Statistical Techniques

● The Debate● Applying ITS to the “real world”

Overview

“Large series of observations made on the same variable consecutively over time” (Shadish, Cook,

& Campbell)

What is Interrupted Time-Series? (ITS)

● The key in ITS designs is knowing exactly when the independent variable was manipulated, or when the intervention was introduced.

● Repeated measurements allow us to identify patterns of change: manipulating the independent variable (introducing an intervention) should cause changes in the level and/or slope of the regression line.

● ITS designs can also demonstrate variability over time (continuous or discontinuous effects, immediate or delayed effects).

Source: http://xkcd.com/418/

• Two basic types of multiple baseline designs (MBDs):o One outcome measured across several cases

at different points in time.o Several outcomes measured on one case,

the intervention applied to one outcome at a time.

• Assumptions?• Advantages?• Disadvantages?

Types of ITS designs

O1 O2 O3 O4 O5 X O6 O7 O8 O9 O10

O1 O2 O3 O4 X O5 O6 O7 O8 O9 X O10 O11 O12

O1 O2 O3 O4 X1 O5 O6 O7 O8 O9 X2 O10 O11 O12

Types of ITS designs

● Internal Validity ○ History○ Instrumentation○ Selection

● Statistical Conclusion Validity

● Construct Validity● External Validity

Major validity threats

http://xkcd.com/605

Autocorrelation

Statistical Techniques for ITS

● ARIMA modeling● C-test● ITS Experiment method● ITSACORR● Latent Growth Modeling● Meta-Analysis

Statistical Techniques for ITS

Is the interrupted time series an experiment?

a debate

● Calling ITS an experiment does not equate it to RCT

● Power of nomenclature● Depends on the purpose of your

study

Summary of the debate

“It might be objected that in community intervention research, one is testing complex, multi-faceted interventions, often involving a series of efforts, and that it will be difficult to sort out which activities contributed to any changes that are observed. […] Practical experience suggests, however, that one emerges from the first effort with a good deal of clarity about how to streamline the intervention”

(Biglan 2000:40)

Systematic Replication

“Effect of pay for performance on the management and outcomes of hypertension in the United Kingdom: interrupted time series study”

Article Reading

What are some of the threats to validity?

How would you improve the study?

What are the implications of these “null” findings?

Article Discussion Questions