Guidelines for Evaluating Surveillance Systems

Disease Control Priorities in Developing Countries. 2nd edition.

The Epidemiology of Obesity
Another aspect of timeliness is the time required for the identification of trends, outbreaks, or the effect of control measures. The behavioral risk factor surveys: The objectives of the system define a framework for evaluating the specific components. A surveillance system can also be useful if it helps to determine that an adverse health event previously thought to be unimportant is actually important. However, because passive surveillance depends on people in different institutions to provide data, data quality and timeliness are difficult to control. Mean waist circumference increased by 10 cm in men and 17 cm in women.

Introduction

Surveillance Systems

They further project that, by , approximately 2. The change over time in adult obesity prevalence was calculated for 28 countries that have two or more nationally representative surveys recorded in the Global Database on BMI. Overall, most countries have rising trends of obesity. Only two of the 28 countries showed a falling trend in the prevalence of obesity in men Denmark and Saudi Arabia , and five of the 28 countries showed a falling trend in the prevalence of obesity in women Denmark, Ireland, Saudi Arabia, Finland, and Spain.

Obesity is caused by a complex interaction between the environment, genetic predisposition, and human behavior. Environmental factors are likely to be major contributors to the obesity epidemic. It is certain that obesity develops when there is a positive imbalance between energy intake and energy expenditure, but the relative contribution of these factors is poorly understood.

Evidence supports the contribution of both excess energy intake and decreased energy expenditure in the obesity epidemic. In addition to environmental factors, there is genetic predisposition to obesity.

These SNPs have modest effects on an individual susceptibility to common forms of obesity, but due to their high frequency, they can have a large contribution to obesity on the population level. In a GWA study of 38, patients, Frayling et al. There is growing recognition that social networks may have an important role in the obesity epidemic. The association was smaller among siblings and spouses: Obesity is associated with an increased risk of death.

Obesity is also associated with increased risk for numerous chronic diseases, including diabetes, hypertension, heart disease, and stroke. Because of the increased risk of death and the increased risk of costly chronic diseases associated with obesity, the obesity epidemic places a large financial burden on the economy. The prevalence of obesity in the U.

The epidemic of obesity is not limited to the U. Obesity is affected by a complex interaction between the environment, genetic predisposition, and human behavior. It is associated with an increased risk of numerous chronic diseases, from diabetes and cancers to many digestive diseases. In addition, the obesity epidemic exerts a heavy toll on the economy with its massive healthcare costs.

The problem of overweight and obesity has therefore emerged as one of the most pressing global issues that we will face during the next several decades, and demands attention from the healthcare community, researchers, and policy makers. The implications for gastrointestinal health care providers is readily apparent and will be addressed in the text of this entire issue of Gastroenterology Clinics.

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National Center for Biotechnology Information , U. Gastroenterol Clin North Am. Author manuscript; available in PMC Mar 1. A surveillance system is a series of surveys conducted again and again to monitor long-term trends in public health. It is used to examine public health issues across several years, to track the trends, compare health among groups of people, and determine whether something is improving or worsening for a specific group of people.

The survey combines interviews and physical examinations. Skip directly to search Skip directly to A to Z list Skip directly to navigation Skip directly to page options Skip directly to site content. Enter Email Address What's this? Related Topics Diabetes Nutrition. Recommend on Facebook Tweet Share Compartir. Youth Risk Behavior Surveillance System YRBS YRBSS measures the prevalence obesity and monitors six types of health-risk behaviors that contribute to the leading causes of death and disability among youth and young adults, including unhealthy dietary behaviors and physical inactivity.

In assessing PVP, primary emphasis is placed on the confirmation of cases reported through the surveillance system. Its effect on the use of public health resources can be considered on two levels. At the level of an individual case, PVP affects the amount of resources used for case investigations.

For example, in some states every reported case of type A hepatitis is promptly investigated by a public health nurse, and family members at risk are referred for prophylactic treatment with immune globulin. A surveillance system with low PVP--and therefore frequent "false-positive" case reports--would lead to wasted resources. The other level is that of detection of epidemics. A high rate of erroneous case reports may trigger an inappropriate outbreak investigation.

Therefore, the proportion of epidemics identified by the surveillance system that are true epidemics is needed to assess this attribute. Calculating the PVP may require that records be kept of all interventions initiated because of information obtained from the surveillance system. A record of the number of case investigations done and the proportion of persons who actually had the condition under surveillance would allow the calculation of the PVP at the level of case detection.

Personnel activity reports, travel records, and telephone logbooks may all be useful in estimating the PVP at the epidemic detection level. PVP is important because a low value means that a non-cases are being investigated and b epidemics may be mistakenly identified. PFalse-positiveP' reports may lead to unnecessary intervention, and falsely detected PepidemicsP' may lead to costly investigations and undue concern in the community.

A surveillance system with high PVP will lead to fewer wild-goose chases and wasted resources. An example of a surveillance evaluation that examined PVP was reported by Barker et al.

They reviewed hospital charts to determine the proportion of persons admitted with a diagnosis of stroke who had the diagnosis confirmed 6. The PVP for a health event is closely related to the clarity and specificity of the case definition. Good communication between the persons who report cases and the receiving agency also can improve PVP. The PVP reflects the sensitivity and specificity of the case definition and the prevalence of the condition in the population Table 1.

The PVP increases with increasing specificity and prevalence. A surveillance system that is representative accurately describes a the occurrence of a health event over time and b its distribution in the population by place and person. Representativeness is assessed by comparing the characteristics of reported events to all such actual events. Although the latter information is generally not known, some judgment of the representativeness of surveillance data is possible, based on knowledge of:.

Characteristics of the population--e. Natural history of the condition--e. Multiple sources of data--e. Representativeness can be examined through special studies that. Quality of data is an important part of representativeness. Much of the discussion in this document focuses on the identification and classification of cases. However, most surveillance systems rely on more than simple case counts. Information commonly collected includes the demographic characteristics of affected persons, details about the health event, and notification of the presence or absence of potential risk factors.

The quality and usefulness and representativeness of this information depends on its completeness and validity. Quality of data is influenced by the clarity of surveillance forms, the quality of training and supervision of persons who complete surveillance forms, and the care exercised in data management. A review of these facets of a surveillance system provides an indirect measure of quality of data. Examining the percentage of unknown or blank responses to items on surveillance forms or questionnaires is straightforward.

Assessing the reliability and validity of responses would require such special studies as chart reviews or re-interviews of respondents. In order to generalize findings from surveillance data to the population at large, the data from a surveillance system should reflect the population characteristics that are important to the goals and objectives of that system. These characteristics generally relate to time, place, and person.

An important result of evaluating the representativeness of a surveillance system is the identification of population subgroups that may be systematically excluded from the reporting system.

This process allows appropriate modification of data collection and more accurate projection of incidence of the health event in the target population. For example, an evaluation of reporting of hepatitis in a county in Washington State suggested that cases of type B hepatitis were under-reported among homosexual males and that cases of type non A-non B hepatitis were under-reported among persons given blood transfusions.

The importance of these risk factors as contributors to the occurrence of these diseases was apparently underestimated by the selective under-reporting of certain types of hepatitis cases 9. Errors and bias can make their way into a surveillance system at any stage. Because surveillance data are used to identify high-risk groups, to target interventions, and to evaluate interventions, it is important to be aware of the strengths and limitations of the information in the system.

So far the discussion of attributes has been aimed at the information collected for cases, but in many surveillance systems morbidity and mortality rates are calculated.

The denominators for these rate calculations are often obtained from a completely separate data system maintained by another agency, e. Thought should be given to the comparability of categories e. The major steps in a surveillance system are shown in Figure 2. The time interval linking any two of the steps in this figure can be examined.

The interval usually considered first is the amount of time between the onset of an adverse health event and the report of the event to the public health agency responsible for instituting control and prevention measures.

Another aspect of timeliness is the time required for the identification of trends, outbreaks, or the effect of control measures. With acute diseases, the onset of symptoms is usually used. Sometimes the date of exposure is used. With chronic diseases, it may be more useful to look at elapsed time from diagnosis rather than to estimate an onset date. The timeliness of a surveillance system should be evaluated in terms of availability of information for disease control--either for immediate control efforts or for long-term program planning.

For example, a study of a surveillance system for Shigella infections indicated that the typical case of shigellosis was brought to the attention of health officials 11 days after onset of symptoms--a period sufficient for the occurrence of secondary and tertiary transmission. This suggests that the level of timeliness was not satisfactory for effective disease control In contrast, when there is a long period of latency between exposure and appearance of disease, the rapid identification of cases of illness may not be as important as the rapid availability of exposure data to provide a basis for interrupting and preventing exposures that lead to disease.

In another time frame, surveillance data are being used by public health agencies to track progress toward the Objectives for the Nation and to plan for the Year Objectives. The need for rapidity of response in a surveillance system depends on the nature of the public health problem under surveillance and the objectives of that system.

Recently, computer technology has been integrated into surveillance systems and may promote timeliness 11, This document covers only the resources directly required to operate a surveillance system. These are sometimes referred to as Pdirect costsP' and include the personnel and financial resources expended in collecting, processing, analyzing, and disseminating the surveillance data. If desired, these measures can be converted to dollar estimates by multiplying the person-time by appropriate salary and benefit figures.

Other resources These may include the cost of travel, training, supplies, equipment, and services e. The application of these resources at all levels of the public health system--from the local health-care provider to municipal, county, state, and Federal health agencies--should be considered. The costs of surveillance systems from two studies are illustrated in Tables 2 and 3 below 7, This approach to assessment of resources includes only those personnel and material resources required for the operation of surveillance and excludes a broader definition of costs that might be considered in a more comprehensive evaluation.

Estimating the overall costs of a surveillance system can be a complex process. The estimates may include the estimation of a indirect costs, such as follow-up laboratory tests or treatment incurred as a result of surveillance; b costs of secondary data sources e.

Costs are often judged relative to benefits, but few evaluations of surveillance systems are likely to include a formal cost-benefit analysis, and such analyses are beyond the scope of this document. More realistically, costs should be judged with respect to the objectives and usefulness of a surveillance system. Examples of resource estimation for surveillance systems operated in Vermont and Kentucky follow. Two methods of collecting surveillance data in Vermont have been compared.

The PpassiveP' system was already in place and consisted of unsolicited reports of notifiable diseases to the district offices or state health department. The PactiveP' system was implemented in a probability sample of physician practices. Each week, a health department employee called these practitioners to solicit reports of selected notifiable diseases.

In comparing the two systems an attempt was made to estimate their costs. The estimates of resources directly applied to the surveillance systems are shown in Table 2. Another example is provided by an assessment of the costs of a surveillance system involving the active solicitation of case reports of type A hepatitis in Kentucky.

Table 3 summarizes the costs of this system. This system found nine more cases than would have been found through the passive surveillance system, and an estimated seven hepatitis cases were prevented through prophylaxis of the contacts of the nine case-patients. These should state whether the system is addressing an important public health problem and is meeting its objectives. The attributes and costs of a surveillance system are interdependent. Before recommending changes in a system, interactions among the attributes and costs should be considered to ensure that benefits resulting from strengthening one attribute do not adversely affect another attribute.

Efforts to increase sensitivity, PVP, timeliness, and representativeness tend to increase the cost of a surveillance system, although savings in efficiency with automation may offset some of these costs However, as sensitivity increases, PVP may decrease. Efforts to increase sensitivity and PVP tend to make a surveillance system more complex--potentially decreasing its acceptability, timeliness, and flexibility. For example, a study comparing health-department-initiated active surveillance and provider-initiated passive surveillance did not improve timeliness, despite increased sensitivity 8.

Evaluating surveillance systems is not easy. There is no perfect system; trade-offs must always be made. Each system is unique and therefore requires a balancing of the effort and resources put into each of its components if the system is to achieve its intended goal. This document has presented guidelines--not absolutes--for the evaluation of surveillance systems.

Attributes have been described that can be examined and evaluated to assess a system's ability to achieve the objectives for which it was designed.

Our goal has been to make the evaluation process more explicit and objective. Suggestions on how we may improve these guidelines would be welcomed. Pertussis and pertussis vaccine: Measuring loss of life, health, and income due to disease and injury. Public Health Rep ; On a method of estimating birth and death rates and the extent of registration. J Am Stat Assoc ;

Obesity Trend in Adults and Children