Descriptive epidemiological studies

Descriptive epidemiology aims to describe the distribution of disease in terms of animal, place and time, as shown below. In a purely descriptive study, no attempt is made to formally investigate reasons for the patterns of disease observed, although hypotheses regarding possible reasons will commonly be generated and developed as a result of these investigations. A description of the different types of descriptive studies is provided elsewhere.

Animal (which animals are affected)

At the most elementary level, an epidemiological study may attempt to quantify the proportion of animals infected with a certain pathogen at a specific point time (known as the 'prevalence' of infection), which could be useful in terms of deciding the burden of a disease within the population at that time. Alternatively, an investigation could be conducted in order to estimate the number of new infections with the pathogen over a certain time period (known as the 'incidence' of infection), which could indicate the spread of infection within the population.Descriptive studies may also be conducted in order to investigate the levels of disease in animals of different ages, sexes, species, etc...

Place (where affected animals are located)

Any epidemiological study entails some consideration of the location of animals, as the population of interest must be determined. Therefore, a study investigating the prevalence of infection of sheep with foot and mouth disease in India is likely to give very different results from the same study conducted in the UK. However, in some cases, the location of affected animals may be explicitly investigated, through the creation of maps of disease occurrence. The ability to visualise and analyse these spatial patterns of disease has increased in recent years due to improvements in computing capabilities, making spatial epidemiology a rapidly growing field, with a number of very useful applications for epidemiological studies.

Time (when cases occur)

Again, the timing of disease is accounted for to some degree in all investigations - meaning that a study investigating the worldwide incidence of Rinderpest in 1900 would give very different results from that in 2010. However, some studies may further investigate and quantify the temporal pattern of disease. One example of this is outbreak investigation, where the number of cases of disease within a population over time may be plotted in the form of an 'epidemic curve'. This can provide useful information regarding trends of disease, and may advise upon the effectiveness of any control measures instigated.

Sampling

A very commonly used type of descriptive studies is the survey, which are very useful in the investigation of the levels of disease in populations, as well as in the crude monitoring of the effect of an intervention or exposure to a risk factor. In these cases, it is of vital importance that the sample is representative of the source population, which means that this population must be clearly defined in terms of animal, place and time, and that forms of selection bias must be minimised. These concepts are covered in other sections.