Measures of Health & Disease

Epidemiology

The measure of distribution and determinants of disease and injuries in human populations

3 Cs of epidemiology

• Counting

• Comparing

• Causal inferences

Compare these sources of data: Routine data vs Ad hoc data

Give 2 examples of Health-Related Quality of Life (HRQoL) measures

EQ-5D

SF-6D

Total fertility rate (TFR)

Average number of children that would be born to a woman over her lifetime

Difference between saying incidence & prevalence

GBD stands for …

Global Burden of Disease

Global Burden of Disease

The burden of disease refers to the physical, emotional, social, and financial impact of diseases on individuals and populations.

Measures health losses from diseases, injuries, and risk factors globally

What do QALYs and DALYs stand for

QALYs - Quality-Adjusted Life Years

DALYs - Disability-Adjusted Life Years

How is QALY calculated & what does it mean

Measures health gain

Uses HrQOL scores

Combine quantity of life & quality of life

• 1 year in perfect health = 1 QALY

• 1 year at half-quality health = 0.5 QALYs

QALY = Quality of life score × Time in health state

Purpose

• Allow comparison across different diseases and interventions

• Used widely in cost-effectiveness analysis

• Central to decision-making bodies such as NICE

• Reflect patient-reported outcomes and lived experience

• Common in high-income regions

How is DALY calculated & what does it mean

Measure health loss

Represent years of healthy life lost due to disease or injury

Two components:

1. Years of Life Lost (YLL) due to premature death

2. Years Lived with Disability (YLD)

Purpose

• Quantify disease burden

• Inform policy & resource allocation

• Compare populations

• Evaluate interventions

• Common in global health

YLL

Years of Life Lost

What does Years of Life Lost measure

A measure of premature mortality

An upper age limit (e.g. 75) is chosen.

A person who dies at the age of 60 contributes 15 YLL (75 − 60 = 15) to the calculation

YLD

Years Lived with Disability

How is Years Lived with Disability measured

Multiply the prevalence or incidence by the Disability Weight.

What are 4 Limitations and Criticisms of QALYs & DALYs

• Place more importance on younger people

• Do not explicitly address equity or fairness

• Reduce complex health experiences to a single number

• Based on population preferences (QALYs)

• Based on expert-derived disability weights (DALYs)

Targeted high-risk strategy vs population shift strategy

2 Measures of test accuracy are sensitivity & specificity. What do these mean?

Sensitivity / Specificity measures true positive / negative rate

Positive predicted value (or negative predicted value) formula

Which of these figures are pulled together to form

PPV

NPV

Sensitivity

Specificity

Which of these depend heavily on prevalence:

• PPV

• NPV

• Sensitivity

• Specificity

PPV & NPV heavily depend on prevalence

Sensitivity and specificity (measures of test accuracy) are unaffected by prevalence

Say there’s 100,000 pregnancies.

100 have the disease; 99,900 are healthy

Results will show:

• 100 who have the disease = 99 diagnosed as TP (99% sens)

• 99,900 who don’t have it – 99900 x 0.01 = 999 diagnosed as FP (99% spec)

What is the Positive Predictive Value?

A. 9%

B. 25%

C. 50%

D. 90%

E. 99%

PPV = TP / (TP+FP)

= 99 / (99+999)

= 99 / 1998

= 9%

Calculate Negative Predictive Value

Say there’s 100,000 pregnancies.

100 have the disease; 99,900 are healthy

Results will show:

• 100 who have the disease = 1 diagnosed as FN (99% sens)

• 99,900 who don’t have it = 99900 x 0.01

= 98901 diagnosed as TN (99% spec)

NPV = TN / (TN+FN)

= 98901 / (1 + 98901)

= 99.9999%

When studying demography, population change may arise because of what 4 factors

• 1. Births

• 2. Deaths

• 3. Migration

• 4. Ageing

Population growth rate formula

3 Policies to increase the number of working-age adults

• Family-friendly policies (e.g. subsidised paternity & maternity leave, tax allowances for parents)

• Encouraging migration

• Increase the state pension age

3 Policies to slow down population growth

• Family planning e.g. subsidised contraception

• Gender equality and education of girls

• One-child policy (China)

What are the 5 stages of demographic transition