In epidemiology, most graphs have two scales or axes, one horizontal and one vertical, that intersect at a right angle. Define any abbreviations or symbols and.Ensure that scales for each axis are appropriate for data presented.Generally, portray frequency on the vertical scale, starting at zero, and classification variable on horizontal scale.Clearly identify variables portrayed (legends or keys), including units of measure.Ensure that a graphic can stand alone by clear labeling of title, source, axes, scales, and legends.
In addition, some best practices for graphics include: Use the graph below to answer this "Quiz Me.When designing graphs, the guidelines for categorizing data for tables also apply. So, the descriptive studies that generate hypotheses are essential. However, it is important to recognize that you can't test a hypothesis unless you have one to test. After an outbreak of Giardia in Milton, MA, a retrospective cohort study was conducted. After the hepatitis outbreak in Marshfield, DPH conducted a case-control study. This means collecting data and analyzing it in order to identify the source. either a case-control study or a cohort study. When this occurs, it is necessary to test the hypotheses by conducting an analytical study, i.e. In other cases, this descriptive information (person, place, and time) helps generate hypotheses about the source, but it isn't obvious what the source is. This is an ideal example, however in reality, most of these epidemics do not produce the classic pattern.įor some outbreaks the descriptive information is all that is needed to figure out the source, and control measures can be undertaken rapidly. The successive waves tend to involve more and more people, until the pool of susceptible people is exhausted or control measures are implemented. Propagated epidemic curves usually have a series of successively larger peaks, which are one incubation period apart. So here transmission is person-to-person, rather than from a common source. (The incubation period for measles averages 10 days with a range of 7-18 days.) One or more of the people infected in the initial wave infected a group of people who become the second wave of infection. The epidemic curve shown below is from an outbreak of measles that began with a single index case who infected a number of other individuals. Propagated (or progressive source) epidemic. This suggests an ongoing source of infection, in this case the Broad Street pump. Cholera has an incubation period of 1-3 days, and even though residents began to flee when the outbreak erupted, you can see that this outbreak lasted for more than a single incubation period. The epidemic curve below is from the cholera outbreak in the Broad Street area of London in 1854 that was investigated by Dr. The down slope of the curve may be very sharp if the common source is removed or gradual if the outbreak is allowed to exhaust itself. This implies that there is an ongoing source of contamination. In a point source epidemic of hepatitis A you would expect the rise and fall of new cases to occur within about a 30 day span of time, which is what is seen in the graph below.Ĭontinuous common source epidemics may also rise to a peak and then fall, but the cases do not all occur within the span of a single incubation period. The incubation period for hepatitis ranges from 15-50 days, with an average of about 28-30 days. The graph above from a hepatitis outbreak is an example of a point source epidemic. Consequently, point source outbreaks tend to have epidemic curves with a rapid increase in cases followed by a somewhat slower decline, and all of the cases tend to fall within one incubation period. Point source outbreaks (epidemics) involve a common source, such as contaminated food or an infected food handler, and all the exposures tend to occur in a relatively brief period. There are three basic types of epidemic curve. The shape of the curve in relation to the incubation period for a particular disease can give clues about the source. An "epidemic curve" shows the frequency of new cases over time based on the date of onset of disease.
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