A set of training materials for professionals working in intervention epidemiology, public health microbiology and infection control and hospital hygiene.
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Cohort studies measuring incidence rates
The computation of effects with incidence rates is similar to calculation of effects from incidence proportions (risk). The incidence rate of disease in exposed (IRe) and unexposed (IRu) can be computed as follows:
A rate difference can be computed:
The relative effect of the exposure on disease occurrence can be measured by computing the rate ratio minus 1.
The rate ratio is:
Breast cancer cases and person-years of observation for women with tuberculosis repeatedly exposed to multiple x-ray fluoroscopies and unexposed women with tuberculosis
Source: Boice & Monson 
One can express the result by saying that the relative effect is 0.86 which would suggest an 86 % increased rate of breast cancer among exposed. One can also express the results by saying that the rate of breast cancer is 1.86 times higher in the exposed cohort than in the unexposed cohort.
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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.
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.
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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