What is an Epi Curve?
An epi curve is a visual display of the onset of illness among cases associated with an outbreak.
You can learn a lot about an outbreak from an epi curve, such as
The outbreak's time trend, that is, the distribution of cases over time
Outliers, that is, cases that stand apart from the overall pattern
General sense of the outbreak's magnitude
Inferences about the outbreak's pattern of spread
The most likely time period of exposure
X and Y Axes
An epi curve is represented by a graph with two axes that intersect at right angles.
The horizontal x-axis is the date or time of illness onset among cases.
The vertical y-axis is the number of cases.
Each axis is divided into equally spaced intervals, although the intervals for the two axes may differ.
Cases
Cases (represented by boxes) are placed along the x-axis according to the date when their earliest symptoms began.
If symptoms for more than one case began on the same date (or time), the boxes are stacked one above the other.
- Epi Curve.png (138.88 KiB) Viewed 1943 times
Time Intervals
The unit of time for the x-axis is based on the incubation period of the disease under investigation and the length of time over which cases are distributed.
As a general rule, the unit of time is set at roughly 1/4 (0.25) of the average incubation period for the illness under investigation.
Average incubation period
multiplied by .25 = Intervals
If the incubation period of an illness is short, the interval on the epi curve might be indicated in hours or even minutes.
View the example below
The incubation period for infection with E. coli O1257:H7 is usually 3–4 days. In this case, you would probably use an interval of one day on the x-axis.
4 days multiplied by 0.25 = 1 day
intervals (4 x 0.25 = 1)