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.



“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.