This design is useful when the risk factor/exposure is transient. For example, cell phone use or sleep disturbances are transitory occurances. Each case serves as its own control, i.e the study is self-matched. For each person, there is a 'case window', the period of time during which the person was a case, and a 'control window', a period time associated with not being a case. Risk exposure during the case window is compared to risk exposure during the control window.
Advantages of Case-crossover
- Efficient – self-matching
- Efficient – select only cases
- Can use multiple control windows for one case window
Disadvantages of Case-crossover
- Information bias – inaccurate recall of exposure during control window (can be overcome by choosing control window to occur after case window)
- Requires careful selection of time period during which the control window occurs (circumstance associated with the control window should be similar to circumstances associated with case window; e.g., traffic volume)
- Requires careful selection of the length and timing of the windows (e.g., in an investigation of the risk of cell phone usage on auto accidents, cell phone usage that ceases 30 minutes before accident unlikely to be relevant to accident)
Analysis of Case-crossover
- Matched case-control analysis
Example of a Case-crossover study
Valent F, Brusaferro S, Barbone F. A case-crossover study of sleep and childhood injury. Pediatrics 2001;107; E23. in Woodward M. Epidemiology: Study Design and Data Analysis. 2nd Ed. London: Chapman and Hall. 2005.
In this Italian case-crossover study of sleep disturbance and injury amongst children (Valent et al., 2001), each child was asked about her or his sleep in the 24 hours before the injury occurred (the case window) and in the 24 hours before that (the control window). Amongst 181 boys, 40 had less than 10 hours sleep on both the days concerned; 111 had less than 10 hours sleep on neither day; 21 had less than 10 hours sleep only on the day before the injury; and 9 had less than 10 hours sleep only on the penultimate day before the injury. The odds ratio (95% confidence interval) for injury, comparing days without and with 10 hours or more sleep, is 2.33 (95% confidence interval; 1.02, 5.79).
You have now completed reading the material for Lesson 7. You are ready to complete the homework.