3: Measures of Association Flashcards
7. Calculate and interpret measures of association including: i. Relative measures such as Risk Ratio (Rate Ratio, Prevalence Ratio, Odds Ratio) ii. Absolute measures such as Risk Difference (Rate Difference, Prevalence Difference) 8. Discuss the difference between relative and absolute measures
what does measure of association ask
Is a disease more common in one (exposed) population
compared with another (unexposed) population?
Independent variables
Exposed or unexposed population
Dependent variables
Outcome of the disease
Measure of association
Used to compare the frequency of disease between specified populations
Reflect the increase in frequency of disease in one population in comparison to another
Measures of disease frequency (risks, rates) can be compared by estimating their ratios or their differences
Measure of association example
One group of cattle are exposed to aflatoxin (exposure of interest) and the other is not.
How can we compare health events between groups?
Ratio measures (relative risk)
Difference Measures (attributable risk)
Ratio measures (relative risk)
Ratios of risk, rates or odds
Measures strength of association between exposure of interest and disease
The higher the ratio the greater the association
If no association the ratio will be close to 1
Uses contingency tables
Prevalence in exposed group =
[a ÷ (a+b)]
Prevalence in unexposed group =
[c ÷ (c+d)]
Risk (prevalence) ratio =
[a ÷ (a+b)] ÷ [c ÷ (c+d)]
Exposure factor
refers to any characteristic that may explain or predict the presence of a study outcome
Outcome of interest
refers to the characteristic that is being predicted
A study was conducted to investigate
differences in the prevalence of BVDV in
beef and dairy cattle.
1. What is the exposure factor?
2. What is our outcome of interest?
- BVDV
- Prevalence of BVDV in population
A random selection of 159 dairy cows
were tested. 33 tested positive to BVDV
A random selection of 506 beef cows
were tested. 72 tested positive to BVDV
1. What is the seroprevalence in dairy
cows?
2. What is the seroprevalence in beef
cows?
Dairy: 20.8%
Beef: 14.2%
What is the risk ratio
1.45
Dairy cows are 1.5 times as likely to be seropositive to BVDV as beef cows
What is the risk ratio
1.45
Dairy cows are 1.5 times as likely to be seropositive to BVDV as beef cows
Difference Measures (attributable risk)
risk in exposed minus risk in unexposed
Measures biological importance of the association
Determines effect of a preventive campaign
Protective factors have negative risk
Cannot be calculated for case-control studies
How to calculate difference in prevalence between groups
[a ÷ (a + b)] - [c ÷ (c + d)]
Risk difference example
Body condition score was compared between adult quenda captured in urbanised environments and bushland
Of 107 urbanised quendas, 58 had a BCS of 4 or 5
(overweight or obese)
Of 151 bushland quendas, 27 had a BCS of 4 or 5
Calculate the prevalence of BCS 4-5 in urbanised quendas
[a ÷ (a+b)]
58 / 107 = 54.2%
Body condition score was compared between adult quenda captured in urbanised environments and bushland
Of 107 urbanised quendas, 58 had a BCS of 4 or 5
(overweight or obese)
Of 151 bushland quendas, 27 had a BCS of 4 or 5
Calculate the prevalence of BCS 4-5 in bushland quendas
[c ÷ (c+d)]
27/151
=17.9%
Body condition score was compared between adult quenda captured in urbanised environments and bushland
Of 107 urbanised quendas, 58 had a BCS of 4 or 5
(overweight or obese)
Of 151 bushland quendas, 27 had a BCS of 4 or 5
Calculate the risk difference (write you answer in words and interpret your results)
Ratio
[a ÷ (a+b)] ÷ [c ÷ (c+d)]
58/107 / 27/151 = 3.02
Diff
[a ÷ (a + b)] - [c ÷ (c + d)]
58/107 - 27/151
54.2 - 17.9 = 36.3
Urban quendas are 3 times as likely to be overweight
or obese than bushland quendas
Urbanised quendas have a 36.3% excess risk of overweight or obesity compared to bushland quendas
Can you estimate prevalence from case-control studies
No but can calculate odds ratio
How to calculate:
ODDS in exposed group
ODDS in unexposed group
ODD RATIO
[a ÷ b]
[c ÷ d]
[a ÷ b] ÷ [c ÷ d] or [a x d] ÷ [b x c]
Association between feeding regimen in lactating cows and herd-level incidence of displaced abomasum or clinical ketosis
Calculate the odds ratio (write you answer in words and interpret your results)
[a x d] ÷ [b x c]
23 x 15 / 5 x 17 = 345 / 85
Odds ratio = 4.0
high-incidence herds had 4 times the odds of feeding Total Mixed Ration compared to low-incidence herds
Absolute measure of association
(LO8)
such as risk differences and incidence rate differences, like risk difference and incidence rate difference quantify excess risk or rate of disease in the exposed group compared to the unexposed group, and this provides a measure of the actual number of people who might be affected and the potential public health impact of the exposure, providing a focus on two key questions:
- How much impact would prevention have?
- How many people would benefit?
Relative measures of association
(LO8)
such as risk ratio and rate ratio quantify the relative risk or rate of disease in an exposed group compared to an unexposed group and tend to be thought of as representing the strength of the association between an exposure and a disease by addressing questions such as:
- How much more likely are exposed persons to develop the outcome than the unexposed?
