Field Epidemiology Manual

A set of training materials for professionals working in intervention epidemiology, public health microbiology and infection control and hospital hygiene.

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Choosing a Reference Group

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Making comparisons is fundamental to epidemiological investigations and studies. We need to compare risk or rates of illness in exposed and unexposed group, or odds of exposure in cases and controls. Without making comparisons with a reference group, we cannot say from data analysis that an association with a given outcome is anything other than spurious. Such a reference group is designated as the control group in case control studies and the unexposed group in cohort studies. For the field epidemiologist, difficulties more often arise in choosing controls for case control studies than in choosing an unexposed group in cohort studies. This section will focus mainly on the former.

In order to define a control group, it is helpful to be clear about who the cases are, in other words,to start with a case definition. The case definition then helps to define the population from which the cases arise, the source population. This population is also the population from which controls should be drawn. The most important principle to follow is that controls should be representative of the source population. Cases can be defined in any way that the investigator decides, but this definition is key to determining the source population of cases, and hence the source population of controls.

There are many ways of choosing controls.This section reviews some of the more common types of controls, their advantages and disadvantages.

reference group, studies

Discussion

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sdesai posted on 9/21/2010 10:45:08 AM:

This chapter is well-written, covers the subject area comprehensively and is easy to read. I have made a few minor editorial changes to the text and below are a few comments and suggestions on specific sections of the chapter.

“Summary”

1. Should biases be mentioned here with a link to the chapter on biases? It is an important consideration when selecting controls and maybe a sentence or two could then really highlight the role of controls.

“Control Selection”

1. In the text you have differentiated controls by random sampling and by matching and I think it would also be clearer if you made this separation at the beginning. Your options could be:

1. Unmatched controls/Randomly selected

a. Population etc

2. Matched controls

a. Neighbourhood etc

This way the text has the same chronological order as the above list.

2. I think it would be a shame not to include control selection in case-case, and case-cross over designs as these are used even if not as commonly as classical case control studies. Their inclusion would complete the picture of control selection.

3. Would it be useful to provide links/references to articles for each type of control selection? For case-case you could use

a. Aiken et al Risk of Salmonella infection with exposure to reptiles in England, 2004-2007. Euro Surveill. 2010; 15(22).

b. McCarthy and Giesecke. Case-case comparisons to study causation of common infectious diseases. Int J Epidemiol 1999; 28:764-8.

For case-crossover you could use

a. Soverow et al. Infectious disease in a warming world: how weather influenced West Nile virus in the United States (2001-2005). Environ Health Perspect. 2009; 117:1049-52.

There is an article by Grimes that might be nice to reference (Grimes DA and Schulz KF. Compared to what? Finding controls for case-control studies. Lancet. 2005;365:1429-33).

“Special considerations in control selection”

1. I think it would be useful to have links to other sections of the manual embedded into the text for “case cohort, traditional case control, density case control”.

“Developing a control definition”

1. I feel it would be more appropriate if this section came straight after the summary page as for me it is more logical to define controls and then determine how to select them.

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