The term interaction indicates that the association between exposure and disease is different depending on the presence or absence of a third factor.
Explain how to calculate the answer step by step
INTRODUCTION
The term interaction indicates that the association between exposure and disease is different depending on the presence or absence of a third factor. In other words, interaction is present when there is significant strata-specific heterogeneity of the observed measures of association or if there is a departure of the observed combined results from the expected (if independent) combined results (typically considering two exposures of interest). If interaction is present, then we cannot present a single, summary measure of association. We must present stratified results.
SECTION A.
A case-control study was conducted to test the hypothesis that exposure to environmental tobacco smoke (ETS) increases risk of nasal cancer in pet dogs. Cases (n=103) were selected from a veterinary teaching hospital during 1986-1990. Controls (n=378) with other forms of cancer were selected from the same study base. Exposure to ETS was evaluated using several methods: 1) by determining the presence of a smoker in the household and 2) by calculating a combined ETS exposure index which included the number of smokers in the house, the number of packs smoked by each smoker, the number of years that each person smoked during the dog’s lifetime, and the proportion of time spent indoors by the dog. All dogs were classified according to their skull shape as either 1) dolichocephalic (long-nosed, e.g., Borzoi or Collie), 2) brachycephalic (short-nosed, e.g., Pugs or Bulldogs), or 3) mesocephalic (e.g., Labrador or German shepherd) that represents an intermediate category that is neither markedly brachycephalic nor dolichocephalic. The following table reports results from the study. Parts of the table were edited out for the purposes of this exercise.
Table 3. Adjusted odds ratios (ORs) for exposure to environmental tobacco smoke in canine nasal cancer, by skull shape, Colorado, 1986-1990 Brachy- and mesocephalic dogs Dolichocephalic dogs Cases Controls Cases Risk Factor Controls exposed exposed OR* exposed exposed OR* (n=50) (n=143) (n=53) (n=235) Smoker None 33 68 1.0 21 138 1.0 Smoker 17 75 32 97 2.0 1.0-4.1 Exposure index None 34 73 1.0 22 141 1.0 0 to <4.5 7 24 11 32 >4.5 46 20 62
A.1 For this scenario, what is the outcome variable? The exposure? The potential interaction variable?
Outcome- nasal cancer
Exposure- environmental tobacco smoke
The potential interaction variable- skull shape
A.2 Examine the possibility of interaction on the multiplicative scale using the heterogeneity approach. Do the odds ratios appear to be homogenous across skull shape categories? (Although the original table showed adjusted odds ratios you only need to calculate crude odds ratios)
M/B case control D case Control
Smoker 17 75 92 Smoker 32 97 129
None S 33 68 101 None S 21 138 159
OR=0.47 50 143 193 OR=2.17 53 235 288
M/B case control D case Control
12+ 8 35 43 12+ 15 43 58
12- 9 40 49 12- 17 54 71
0 33 68 101 0 21 138 159
12-OR=0.5 50 143
12+
OR=0.5
12-
OR=2.1
53 235
12+
OR=2.3
M/B case control D case Control
>4.5 9 46 55 >4.5 20 62 82
<4.5 7 24 31 <4.5 11 32 43
0 34 73 86 0 22 141 163
>4.5
OR=0.4
50 143
<4.5
OR=0.6
>4.5
OR=2.1
53 235
<4.5
OR=2.2
A.3 Now let’s examine presence of interaction using the expected combined effect approach. In doing so we will use only the presence of a smoker in the house as a measure of ETS exposure (top two rows of the table on page 3). Complete the following table.
Brachy/mesoskull
ETS exposure
Cases
Controls
OR
No No 21 138 1.0
No Yes 32 97
Yes No 33 68
Yes Yes 17 75
A.4 What is the effect of brachy-/mesocephalic skull alone (that is, in the absence of ETS exposure)?
OR for brachy-/mesocephalic skull is 3.2
A.5 What is the effect of ETS alone (that is in dogs that are not brachy-/mesocephalic)?
OR for ETS is 2.2
A.6 What is the expected combined multiplicative effect of brachy-/mesocephalic skull and ETS?
the expected combined multiplicative OR is 7
A.7 Compare the expected combined OR with the observed combined OR. What can be concluded regarding the presence and type of interaction?
The expected combined OR is 7 and the observed combined OR is 1.5. There is multiplicative interaction between skull shape and ETS.
Collepals.com Plagiarism Free Papers
Are you looking for custom essay writing service or even dissertation writing services? Just request for our write my paper service, and we'll match you with the best essay writer in your subject! With an exceptional team of professional academic experts in a wide range of subjects, we can guarantee you an unrivaled quality of custom-written papers.
Get ZERO PLAGIARISM, HUMAN WRITTEN ESSAYS
Why Hire Collepals.com writers to do your paper?
Quality- We are experienced and have access to ample research materials.
We write plagiarism Free Content
Confidential- We never share or sell your personal information to third parties.
Support-Chat with us today! We are always waiting to answer all your questions.
