Measures Of Population Impact Flashcards by Isabella Brizzolara

When are measures of population impact particularly useful in epidemiology?

We may have a particular outcome of interest and then some risk factors which are shown to be associated with it. We may have the risk ratios for these risk factors, so we may assume that we should prioritise the risk factor with the greatest risk ratio, however when we look further we may explore measures like the prevalence of the risk factor in the population and the one with the smallest risk ratio may actually have the largest prevalence in the population so the population may benefit mostly from the target of this risk factor

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What is a measure of effect?

A way to compare the frequency of a disease in the exposed and the unexposed

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What is the formula for Risk Ratio or Rate Ratio?

RR = R1/R0

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What is the formula for Risk Difference, what is this also known as?

RD = R1-R0

Also known as the attributable risk in the exposed

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What is the formula for Risk Difference Percent, what is this also known as?

RDP = (R1-R0)/R1

Also known as the attributable fraction in the exposed

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What are measures if impact?

They are ways to compare the frequency of disease in the population compared to the unexposed

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In terms of exposed and unexposed, where will the overall risk in the population fall and why?

Will fall somewhere within the Exposed and the Unexposed groups as the entire population will be made up of these two groups

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Which question can we use measures of impact to answer?

What would happen to the risk of disease within an entire population if the percentage of the population which were exposed to a certain risk factor increased/decreased?

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What are we estimating when we use measures of Impact?

Estimating the expected impact of exposure in the total study population

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What are the two main measures of impact?

Population Attributable Risk

2. Population Attributable Risk Fraction

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What does Population Attributable Risk measure?

The excess risk of disease in the study population that is attributable to exposure

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What does population attributable risk fraction measure?

The percentage of the disease in the study population that is attributable to the exposure

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What are measures of impact effected by?

The strength of an association between a risk factor and an outcome
2 How common a risk factor is within the population

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How do we calculate the population attributable risk or rate?
What is the formula

Find the difference between the risk ( or the rate) in the whole population (r) and the risk (or the rate) in the unexposed (r0)

PAR = R-R0

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Give an example of how we would present the interpretation of the population attributable risk calculations.

In Population X, the exposure was responsible for 2.5 cases of disease out of every 100 people

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What is the difference between population attributable risk and the risk difference?

The risk Difference only compares the exposed to the unexposed whereas the population attributable risk compares the whole population to the unexposed

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What is the Population Attributable Risk Fraction?

The proportion of all cases in the whole population which may be attributed to the risk factor

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How do you calculate the Population Attributable Risk Fraction?

PAF = (r-r0)r

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How would we present our findings of the population attributable risk fraction calculations? Give an example

In the population, X% or X fraction of the diseased cases are attributable to X exposure

What happens to the Population Attributable Risk fraction when the prevalence of the exposure in the population is reduced?
In this situation, what is the driver which has caused the change in PAF?

The proportion of individuals within the entire population which are exposed to the exposure will be reduced as there is less of the exposure
This means that the composition of the entire population in terms of exposure will look more similar to the unexposed group which we are comparing to so the PAF will be decreased

The driver is not the risk, as this hasn’t changed
The driver would be the prevalence of the exposure in the population

In many epidemiological studies, in terms of attributable risk, what are we commonly lacking and why?

We often only take samples of exposed and unexposed groups of individuals so often we don’t know the risk in the whole population which may be attributed to the risk factor

When we don’t know the risk of the whole population which is attributed to the risk factor, which alternative formula can be used to calculate Population Attributable Risk?

PAR = p(r1-r0)

p = prevalence of the exposure in the population (expressed as a proportion) 
r1 = Risk in the exposed 
r0 = Risk in the unexposed

How can we use Risk or Rate Ratios to calculate the Population Attributable Risk Fraction?

PAF = p(RR-1)/(p(RR-1)/1)

How do we calculate Population Attributable Risk Fraction if we don’t know the prevalence of the exposure in the whole population but just amongst the cases?

when is this calculation particularly useful?

PAF = p1(RR-1)/RR

p1 = the prevalence of the exposure amongst the cases

useful in case control studies and also for calculating the adjusted rate or risk ratios accounting for confounders

What are some alternative names for the Population Attributable Risk Fraction?

Population Attributable Fraction Aetiological Fraction Percentage population attributable risk Attributable fraction

What are the two factors that affect population attributable risk and Population attributable risk fractions? What will these affects be dependent on?

1. Strength of the association between the exposure and the outcome 2. The prevalence of the exposure in the population these will be dependent on the specific population being studied.

Introducing hand sanitiser in hospitals will save more lives than if it were introduced in shopping centres. In terms of impact measures, why is this?

In this example, the exposure is exposure to some infection The outcome is mortality. Although the strength of association between the infection and the mortality may be the same in the hospital and the shopping centre, the prevalence of the infection would be much greater in the hospital which would mean that the PAF would be greater.

Which things do we assume when interpreting measures of impact? (5)

- all the association between exposure and outcome is causal - the exposure and frequency of an outcome is measured correctly - Removal of the exposure actually removes the risk - The exposure is actually removable - All other risk factors remain constant

Data - Number of Non smokers = 100,00 Number with heart disease = 34 Number of smokers = 7500 Number with heart disease = 54 Total population = 175,00 Number with heart disease = 88 Question = what percentage of heart disease in Bulgaria is attributable to smoking in adults? 1. Calculate the risk of Heart disease in each group

Risk in Non smokers = 0.0034 (per 100,00) Risk in Smokers = 0.0072 (Per 7,500) Risk in total = 0.005 (per 175,00) express as per 1,000 people 1. 3.4 2. 7.2 3. 5.0

2. calculate the risk ratio for this data

RR = Risk in exposed/risk in unexposed | = 7.2/3.4 = 2.1

3. Calculate the risk difference for this data

``` RD = Risk in exposed - Risk in unexposed RD = 7.2-3.4 = 3.8 ```

4. Using this data alone, which measures of impact are we able to calculate?

1. Population Attributable Risk | 2. Population Attributable Risk Fraction

5. Calculate and interpret the Population Attributable Risk for the data

PAR = (r-r0) = 5.0-3.4 = 1.6 | In the total population, 1.6 cases of heart disease are attributable to smoking

6. Calculate the Population Attributable Risk Fraction for the data

PAF = (r-r0)/r = (5-3.4)/5 = 32%

This study was based in Bulgaria, would be expect the PAF to be the same in Ethiopia?

No, because we should assume that, as they are different populations, the prevalence of smoking would vary between these two populations

It was found that in Ethiopia compared to Bulgaria, the prevalence of smoking is reduced, what would this do to the PAR and the PAF?

Both will decrease

As we estimated a PAF of 32% for Bulgaria, could we therefore conclude that 32% of heart disease in Bulgaria is attributable to smoking?

Just based on this data, we may want to be sceptical | we want more evidence

Why do we not conclude that the PAF is actually a true number within a population?

we have to look at the 5 assumptions we made at the start, many of these may not actually be true - the association may not actually be fully causal, there may be other factors influencing this association - the measurement of the exposure and frequency may not be reliable or accurate - removal of the exposure may not actually remove all of the risk associated - it may not actually be possible for the exposure to be completely removed - other risk factors may not remain constant

Children who are not breastfed have a 3 times higher risk of diarrhoea compared to those exclusively breastfed What % of diarrhoea disease is attributable to not breastfeeding in a country where 30% of children are not breastfed? How would we do this?

1. Calculate PAF using a suitable formula for the data provided

In this example, which PAF formula is suitable?

PAF = p(RR-1)/p(RR-1)+1 p = how many people are exposed in the entire population

Calculate the PAF from the example scenario (diarrhoea) | Interpret this

PAF = p(RR-1)/p(RR-1)+1 0. 3(3-1)/0.3(3-1)+1 0. 6/1.6 0. 375 = 37.5% In this country, 37.5% of diarrhoea cases can be attributed to not breastfeeding (assuming all assumptions are true)

Found that PAF for diarrhoea when not breastfeeding is 37.5% whereas the PAF for diarrhoea when a child has a vitamin A deficiency is 35.1%. How would you decide which of these two health interventions to target?

Look back over the assumptions made to calculate PAF

"How many intravenous drug users currently share needles?" what epidemiological measure is being measured here?

Measure of disease frequency - Prevalence

"By how many times does sharing a needle increase the risk of HIV infection amongst intravenous drug users?" which epidemiological measure is being measured here?

Measure of effect - Risk ratio

"What would the impact be of eliminating needle sharing among intravenous drug users on the incidence risk of HIV infection in the entire population?" which epidemiological measure is being measured here?

Measure of Impact - PAR or PAF

"By what percentage would HIV incidence amongst intravenous drug users who share needles fall if sharing needles was eliminated as a practice in this group?" which epidemiological measure is being measured here?

Measure of effect - Risk Difference Percent