JiscMail Logo
Email discussion lists for the UK Education and Research communities

Help for ALLSTAT Archives


ALLSTAT Archives

ALLSTAT Archives


allstat@JISCMAIL.AC.UK


View:

Message:

[

First

|

Previous

|

Next

|

Last

]

By Topic:

[

First

|

Previous

|

Next

|

Last

]

By Author:

[

First

|

Previous

|

Next

|

Last

]

Font:

Proportional Font

LISTSERV Archives

LISTSERV Archives

ALLSTAT Home

ALLSTAT Home

ALLSTAT  December 2018

ALLSTAT December 2018

Options

Subscribe or Unsubscribe

Subscribe or Unsubscribe

Log In

Log In

Get Password

Get Password

Subject:

Case-Control Studies – Direction of Relationship

From:

Kim Pearce <[log in to unmask]>

Reply-To:

Kim Pearce <[log in to unmask]>

Date:

Mon, 10 Dec 2018 14:30:25 +0000

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (50 lines)

Hello everyone,

I hope that someone can shed some light on the topic below.

In a case-control study, I would like to ask your opinion about the direction of the relationship between the risk factor and the outcome.

Let us consider, initially, one risk factor.

NON MATCHED STUDY

Say we have a non matched case-control study where we enrol controls without regard to the number or characteristics of the cases.   We will assume that the cases have lung cancer and the controls do not have lung cancer.  Our risk factor is “does the subject smoke” (yes or no).
Case-control studies are ‘retrospective’ i.e.  in this study, we sample subjects whose lung cancer status is already known and then establish how many in each of the outcome groups smoked.  Thus, in my way of thinking, the odds ratio would be expressed as “the odds of *smoking* when the subject has lung cancer compared to the odds of *smoking* when the subject has not lung cancer”.
If, in a hypothetical example, we had 50 cases and 100 controls and 40 of the cases smoked and 30 of the controls smoked, then the “the odds of *smoking* when the subject has lung cancer compared to the odds of *smoking* when the subject has not lung cancer” = [40/10]/[30/70] = 9.33.
(Admittedly, this is mathematically equivalent to  the “the odds of *lung cancer* when the subject smokes compared to the odds of *lung cancer* when the subject does not smoke” = [40/30]/[10/70] = 9.33)

MATCHED STUDY
Say we now consider a 1:1 matched case-control study where we enrol each control based on some characteristics of a case i.e. we match upon potential “confounding” variables e.g. sex and gender.   We’ll say  that we matched the cases and controls for sex and gender in the following hypothetical example. There were 45 pairs for which the lung cancer patient but not the non lung cancer patient was a smoker and there were 24 pairs where the non lung cancer patient but not the lung cancer patient was a smoker.  Thus the odds ratio for these data is 45/24 = 1.875 i.e. if you have lung cancer you’re 1.875 times more likely to be a smoker than if you don’t have lung cancer (even when we control for sex and gender).

MODELLING
Now for a non-matched case-control study, we would carry out an unconditional logistic regression and for a matched case-control study we would carry out a conditional logistic regression.  Say our primary risk factor was smoking and our potential confounders were sex and gender, then (i) the sex and gender variables would appear in the unconditional logistic regression model for an adjusted estimate of “smoking” and (ii) sex and gender would *not* appear in the conditional logistic regression model as we had already matched the cases and controls as regards these two variables. 

Q1My question is regarding the interpretation of the “odds ratios”…..as I have touched upon above, for a case-control study, we already have the two-group outcome (e.g. lung cancer and no lung cancer) and we are looking to see how many in each of the two groups have the a risk factor (e.g. smoking)….so really the odds ratio is the “odds of the risk factor (smoking) when we have a “case” (lung cancer) compared to odds of the risk factor (smoking) when we have a “control” (non lung cancer)”…..however, when interpreting odds ratios  for a case-control study (in an unconditional or conditional logistic regression model scenario),  it *is* actually perfectly correct to interpret the odds ratio as the “odds of the case (lung cancer) when we have the risk factor (i.e. “smoking”) compared to the odds of the case (lung cancer) when we do not have the risk factor (i.e. “not  smoking”)….(controlling for all other variables in the model). That is correct isn't it? This seems to tie in with all of the examples I have looked at for unconditional or conditional logistic regression.

Q2 When we are doing a matched case-control study, the variables that we use for matching (i.e. the confounders) are those which (in a non matched case-control study) are thought to be associated with the risk factor of interest (smoking) and the outcome (lung cancer).  In  a non matched case-control study say the analysis reveals a relationship between smoking and lung cancer, but there may be more with lung cancer who are older (compared to non lung cancer) and it could be that a higher proportion of smokers are elderly (compared to the non smokers)....therefore  it would make sense in this scenario to match each case-control pair as regards age (as failure to do so would result in a biased estimate of the effect of smoking).  I am correct in my thoughts aren’t I? 

Many thanks for your views on the above.  I am sure that I am correct in my assumptions but I thought I'd double check.

Kind Regards,
Kim


Dr Kim Pearce PhD, CStat, Fellow HEA
Senior Statistician
Faculty of Medical Sciences Graduate School 
Room 3.14
3rd Floor 
Ridley Building 1
Newcastle University
Queen Victoria Road 
Newcastle Upon Tyne 
NE1 7RU

Tel: (0044) (0)191 208 8142

You may leave the list at any time by sending the command

SIGNOFF allstat

to [log in to unmask], leaving the subject line blank.

Top of Message | Previous Page | Permalink

JiscMail Tools


RSS Feeds and Sharing


Advanced Options


Archives

March 2024
February 2024
January 2024
December 2023
November 2023
October 2023
September 2023
August 2023
July 2023
June 2023
May 2023
April 2023
March 2023
February 2023
January 2023
December 2022
November 2022
October 2022
September 2022
August 2022
July 2022
June 2022
May 2022
April 2022
March 2022
February 2022
January 2022
December 2021
November 2021
October 2021
September 2021
August 2021
July 2021
June 2021
May 2021
April 2021
March 2021
February 2021
January 2021
December 2020
November 2020
October 2020
September 2020
August 2020
July 2020
June 2020
May 2020
April 2020
March 2020
February 2020
January 2020
December 2019
November 2019
October 2019
September 2019
August 2019
July 2019
June 2019
May 2019
April 2019
March 2019
February 2019
January 2019
December 2018
November 2018
October 2018
September 2018
August 2018
July 2018
June 2018
May 2018
April 2018
March 2018
February 2018
January 2018
December 2017
November 2017
October 2017
September 2017
August 2017
July 2017
June 2017
May 2017
April 2017
March 2017
February 2017
January 2017
December 2016
November 2016
October 2016
September 2016
August 2016
July 2016
June 2016
May 2016
April 2016
March 2016
February 2016
January 2016
December 2015
November 2015
October 2015
September 2015
August 2015
July 2015
June 2015
May 2015
April 2015
March 2015
February 2015
January 2015
December 2014
November 2014
October 2014
September 2014
August 2014
July 2014
June 2014
May 2014
April 2014
March 2014
February 2014
January 2014
December 2013
November 2013
October 2013
September 2013
August 2013
July 2013
June 2013
May 2013
April 2013
March 2013
February 2013
January 2013
December 2012
November 2012
October 2012
September 2012
August 2012
July 2012
June 2012
May 2012
April 2012
March 2012
February 2012
January 2012
December 2011
November 2011
October 2011
September 2011
August 2011
July 2011
June 2011
May 2011
April 2011
March 2011
February 2011
January 2011
December 2010
November 2010
October 2010
September 2010
August 2010
July 2010
June 2010
May 2010
April 2010
March 2010
February 2010
January 2010
December 2009
November 2009
October 2009
September 2009
August 2009
July 2009
June 2009
May 2009
April 2009
March 2009
February 2009
January 2009
December 2008
November 2008
October 2008
September 2008
August 2008
July 2008
June 2008
May 2008
April 2008
March 2008
February 2008
January 2008
December 2007
November 2007
October 2007
September 2007
August 2007
July 2007
June 2007
May 2007
April 2007
March 2007
February 2007
January 2007
2006
2005
2004
2003
2002
2001
2000
1999
1998


JiscMail is a Jisc service.

View our service policies at https://www.jiscmail.ac.uk/policyandsecurity/ and Jisc's privacy policy at https://www.jisc.ac.uk/website/privacy-notice

For help and support help@jisc.ac.uk

Secured by F-Secure Anti-Virus CataList Email List Search Powered by the LISTSERV Email List Manager