Biology Reference
In-Depth Information
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Figure 1.3
Demonstration of cross-correlation function (CCF)—decrease in similarity of two curves
decreases the overall CCF magnitude. (From Suyama et al. 2003. Surveillance of infectious
disease occurrences in the community: an analysis of symptom presentation in the emergency
department.
Acad Emerg Med
10 (7): 753-63.)
0.25) ahead of regional influenza activity. Hence, at this peak correlation,
emergency room telephone triage was on average 17.5 days ahead of all influ-
enza activity.
Magruder (2003) found that after controlling for day-of-week and holiday
effects, the CCF peaked between 0.86 and 0.93 for OTC sales and physician
diagnoses. At this peak correlation, OTC sales were on average, 2.8 days
(range 2-7 days) ahead of physician diagnoses based on two influenza sea-
sons (Magruder 2003).
Johnson et al. (2004) investigated the correlation using the CCF between
influenza article access on the Internet and CDC surveillance data. Although
there was a moderately strong correlation between Web access and influenza
reports (range 0.71-0.80), the timeliness of this method was variable, and hence
the authors could not draw any strong conclusions (Johnson et al. 2004).
One pediatric study used both the CCF and an exponentially weighted
moving average (EWMA) to determine the timeliness of free-text chief
