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In addition to these inherent limitations, surveillance, as practiced in humanitarian emergencies, often has several problems.
Regardless of how well trained and dedicated, health workers often fail to submit the data requested of them. They may not complete reporting forms or not remember to submit them. If only 50% of cases of a certain disease are reported, the calculated incidence rate will be only 50% of the real rate in the population. If you assume that all cases are reported, you may substantially underestimate the importance of that disease.
Remember, reporting diseases is probably not in a health care worker's job description, they are not being paid for it, and they are not being evaluated on how well they do it. It is up to the public health authorities to motivate health workers by providing feedback showing how the data collected can help their work and improve the health of the population.
When analyzing surveillance data, it may be a good idea to maintain a high degree of scepticism. Always assume that many cases of disease are not reported. If a surveillance system is stable, so that the same proportion of actual cases of disease are reported to surveillance, then the changes over time tell you much more than the absolute incidence rates. If fact, it may not be possible to rely on the incidence rates calculated from incomplete surveillance data.
Completeness of reporting
A study in the United States in the early 1990s demonstrated that less than 10% of all cases of viral hepatitis were reported to public health authorities. And this was in a country with a relatively well-developed health care and public health system.