3. Gathering and Using Data

3.1. Use of Rates: Vital Signs of a Community

Rates are a fraction representing numbers of cases of specific conditions over the number of people in a specific population group, multiplied by an even number depicting the population at risk (Box 2). Rates facilitate the comparison between the reality of one community and that of others. They also help assess through time the success of interventions in a given population.

BOX 2: The “patient” of preventive medicine
  • Groups, not individual patients
  • “Vital signs” = Rates of disease
  • Rates = persons x even number persons at risk
  • Even number: represents the size of the population (1,000, 10,000 or 100,000)

Determining rates is a skill that many clinicians do not use on a daily basis, but it is critical to understanding health problems in a community. Without this data, scarce resources will not be used rationally for the good of the community. This is particularly critical in post-disaster scenarios where resources, such as time, are more constrained than usual.

To obtain rates, one must have both a numerator and a denominator. The numerator is the number of cases of a specific type of problem, and the denominator is the number of people in the community who are at risk for the problem. The resulting number can be reported as a fraction, a percent, or a rate. All of these convey useful information and can be converted from one to another. Using a rate is the way that most public health practitioners “speak” to each other and is probably the most useful (Box 3 and 4).

BOX 3: Rates
  • Example: Children younger than 5 years of age with diarrhea

Town A: 304 cases of diarrhea

Town B: 1054 cases of diarrhea

Which town has more problems with diarrhea?

BOX 4: Rates: a numerator and a denominator
  • Town A: 1597 children <5 years of age
    • Rate: \( 304/1597 * 10000 \) = 1904
  • Town B: 12,818 children <5 years of age
    • Rate: \( 1054/12,818 * 10000 \) = 822

Rates of diarrhea per 10,000 children younger than 5 years.

The value of a rate is dependent on the quality of the data that go into its creation. For accurate numerators, cases must be defined clearly so that a busy clinician can easily categorize problems. For example, a typical case definition would be 3 or more watery stools for a diarrhea case. Consistency in defining cases is key to ensuring the comparability of rates from different areas or following them over time.

Equally important to determining accurate numerators are accurate and descriptive denominators. For this, basic demographic information is needed, such as the total number of people affected in the community and the population structure, including gender breakdown and number of people in specific age groups. 

In a disaster scenario, the simplest way of subdividing groups by age is to classify them under 5 years, 5-15 years and over 15 years. It might be useful to subdivide this last group into 15 to 60 and over 60.

The most critical rates to follow after a disaster are mortality (death) rates. The daily crude mortality rate (CMR) is determined by taking the total number of deaths in a population (community), dividing it by the total number of people in that population, and multiplying that number by 10,000 (Box 5).

BOX 5. Crude mortality rate (CMR)

(Total number of deaths in a group ÷ Total number of persons in this group)  × 10,000

Expressed as deaths per 10,000 persons per day. The objective is <1/10,000/day

For example, if a community has a population of 15,955 and it experiences 49 deaths in 7 days, the CMR will be 49 / 15,955 x 10,000 = 30.7 deaths per 10,000 people in one week. To arrive at the daily CMR, which is the international standard for gauging disaster severity and effectiveness of response, divide this number by 7 to get a daily CMR of 4.4 deaths per 10,000 people per day.

Attack rates are incidence rates; i.e., they reflect the number of new cases in a given population.

The mortality rate of children under 5 years, i.e. the number of deaths in children younger than 5 years, is another important measure to assess the severity of a disaster and the capacity for response. It is important not only because it shows the effects of the disaster on children, but also because children are the most vulnerable members of society. This age group is usually called the “sentinel population,” because changes will become evident sooner than in other age segments. Health-care workers should worry when the mortality rate reaches 2 deaths/10,000 children age <5/day.The situation is considered severe when this rate mounts to 4 deaths/10,000 children age <5/day.

Attack rates are also usually utilized during disaster situations. These rates express the relation between the number of newly diseased persons (cases) and the total population at risk. Attack rates are incidence rates, i.e., they reflect the number of new cases in a given population. On the other hand, prevalence rates measure the proportion of cases of different diseases in a given population. They express the specific weight of a given disease with relation to the aggregate of all diseases, and allow establishing priorities in the management of diseases and the use of human resources. However, in contrast to incidence rates, prevalence rates do not reflect the risk of an epidemic.