Discussion: Biases in Epidemiological Studies


The Field Epidemiology Manual provides a set of core training materials reflecting the essential competencies required by intervention epidemiologists.

Discussion: Biases in Epidemiological Studies

  • The definitions of different types of bias should be reconciled with the definitions included in the ECDC internal lexicon (ie that developed by Laszlo)

  • Good idea but we have to ask how to get access to these terms

  • Vladimir will check with Laszlo availabllity of the terminology server and a set of already defined terms.

  • I suggest the following structure for this chapter:

    1. Concepts and definitions - including examples and brief discussion of potential impact on studies (the definitions should be the same as, or compatible with, those to be found in the ECDC data dictionary)

    2. (a) How to avoid bias through study design

    2. (b) Checklist to be used by study designers for identifying potential sources of bias

    3. How to identify and manage bias in analysis and interpretation

    4. EPIET Slide Set

    5. Self-assessment questions

    As discussed, sections 2 and 3 may also be relevant to Chapter 9

  • I agree with this proposal but like to add some learning objectives like: ' After reading this chapter you will be able to ' 1) understand the different types of biases (etc....) 2) etc...

  • Current status of the terminology services - answer from Laszlo:

    The current version as such is running behind the firewall, within our internal network. However, as it has a machine interface, there could be a method to feed a list in the wiki with proper terms queried from the Term Server and updated regularly. This will need some work to tag our categories with the attribute `is required by fem wiki`.  That way the users of the wiki could consult the derived lists in the wiki itself.

    For this we have discuss what term sets are needed. Now a systematic audit of all systems / applications is ongoing to check their information objects regarding the core metadata standard implementation. We could join these two tasks in a single work process.

    *** I hope this answer is promising. However at present we have to refer to books or other ready to use services.

  • Hi Aileen

    Reading through the chapter again, I was wondering if it could be useful to add the 2x2 tables that we have as examples in the EPIET Powerpoint lecture for each form of bias. What do you think?




  • Hi Arnold,

    thanks for the suggestion! and apologies for the delayed reply. I have added the 2x2 tables from the EPIET lectures we had in Menorca (these still need some editing, up/down arrows put in etc), and some other examples also from the lectures, and from Rothman and other books/ articles.

    Please let me know if you would like an changes made to the additions to the text/ edits of the text.

    All the best,


  • Hi,

    This is a question for Arnold & Vladimir :-).

    In the original text of my chapter 'biases in epi studies' , it says that random error will be dealt with in a separate chapter. I realise that not all chapters are finished yet, but was wondering which chapter will deal with random error, confidence intervals, p-values etc?

    Best wishes,


  • This chapter is well written, accurate and clear overall. I have made some editing suggestions. (My machine crashes each time I am in the information bias section and try to edit, so I havent edited from there on - not sure if this is a local, or global issue)

    The definition of bias could perhaps be introduced earlier. There is another broader definition used by Daly which defines bias as "any factor or process that tends to produce results or conclusions that differ systematically from the truth" (Interpretation and uses of medical statistics, Leslie E Daly, Geoffrey J Bourke, 5th edition, 2007). Perhaps this could be used, and then lead on to the more focused definition given later

    Also, has what we mean by an association been defined elsewhere in the text?  If not, suggest that this is discussed too.

    I have suggested including outcome as well as disease wherever its mentioned in the first section, as we are not only interested in diseases, rather outcomes too.

    In the non-response bias section, I think that it would be helpful to have some text to explain the scenarios more fully

    In the preventing non-response bias section one additional method to achieve high response rates (as well as incentives) is to make it easy to contribute, eg by using questionnaires that are not too long, and dont take too much time to complete

  • These sound like good points to me. There may be a challenge in finding the right balance between the amount of text per screen, and keeping the overview of the topic.

    Still, it is in my view worth to try and put these concepts in the chapter. Derval, would you care to try an attempt in editing? This will also allow us to test how that part of our process will work out once the WIKI is open to the public. We will definitely get users who start modifying the texts, and that is exactly what we want.

    So if you feel inspired, please go ahead, and we can check out how that works for the chief-chapter-editor :)

  • Hi Aileen,

    sorry for the tardy reply, and we have already touched upon the solution in our last teleconference. There will not soon be a 'random error' chapter, as it seems to have been described sufficiently for the time being in the lines within the Bias chapter.

    However, the chapter on 'P-value' would in my view indeed be a useful next priority.

    Perhaps an idea for the current facilitators in Menorca? :)

  • Many thanks Derval for the review of the chapter, and Arnold for additional comments!

    Derval, I was also having the same problem with the Information Bias page - it kept crashing every time I tried to edit it - it didn't seem to be able to be fixed, so I have written the page again, and if you would like to take another look again, that would be great!

    I have incorporated some of your other suggestions already in case you want to review those:  

    - I have added Leslie Daly's definition of bias to the existing paragraph, and added a definition of an association in the 2nd paragraph in that page (feel free to adapt it if you would like)

    - disease/outcome is now throughout the text

    - I added your suggestion about the questionnaire to the 'preventing non-response bias' section

    I would be happy for you to try some editing, as Arnold suggested, and if you feel inspired :-)  

  • Aileen + Derval,

    Great work!  Please do try editing with Internet Explorer...in case the browser freezing/crashing problem is stemming from the use of Firefox.

    Another message to follow shortly, Aileen, about your table formatting query.

    Thank you,


  • Dear Lisa,

    Many thanks for your reply!  

    I have been using Internet Explorer all the time as a browser - I mentioned this also at the last telco. I really don't think it;s a browser issue, else why would it only happen on one page?? and not on others while editing.

    I did wonder whether it could be related to the amount of editing on the page? e.g. if many changes are made to a page, could that be causing the problem?  

    if so, I would be concerned that this would cause issues when the wiki goes live......  

    I would be grateful if you & Martin could take a look at this. Perhaps you could also check with the others whether any of them are having problems with editing formatting, etc. as I also find that sometimes I have changed things & this is not reflected in the View page afterwards.

    Many thanks,