Introduction to Epidemiology

Professor Iain Crombie

Course outline

13 days 11 lectures and tutorials 1 revision session 1 exam

plus private study

aim: to make you like an epidemiologist

Detailed course outline: identifying causes of disease

disease distributions and measuring health descriptive epidemiology, demography, standardisation

interpreting study findings bias, p-values, confounding

epidemiological designs survey, ecological, case-control, cohort, RCT pitfalls in study design and conduct

assess causality how do we know what causes disease how do we assess strength of evidence

A thinking epidemiologist

able to interpret the studies published in medical journals

Teenage pregnancy and sunshine: causal?

Teaching philosophy

given all the necessary explanation you have to use facts not just remember deep understanding is necessary

lots of practice in data interpretation if you don’t follow, ask

the course assessment is about study design

• key features

• identifying flaws/ strengths

interpreting data assessing causality

Assessment assignment 1 15%

data interpretation

assignment 2 15% interpretation of data from a paper

exam (3hrs) 70% section 1 data interpretation section 2 short notes section 3 essay question section 4 interpretation of an abstract

Learning objectives: today

You should be able to define epidemiology describe descriptive epidemiology define measures of disease frequency outline some key concepts in epidemiology think about data

What is epidemiology?

The study of epidemics?

The study of diseases?

The study of diseases of the skin?

Something scientists and academics use to confuse other people?

Definition of epidemiology

“The study of the distribution and determinants of health related states or events in specified populations”

Unpacking that definition

Distribution/ frequencywho, where, when

Determinantswhy

Health related statesdiseases or symptoms or..

Specified populationsread carefully

Definition of epidemiology

“The study of the distribution and determinants of health related states or events in specified populations and the application of this study to control health problems” - James Last A Dictionary of Epidemiology

Examples of determinants

Diseases• Lung cancer• Mesothelioma• Childhood leukaemia• Cervical cancer• Liver cancer• Coronary heart disease

Some causes• smoking, radon• asbestos• intra-uterine X-rays• human papilloma virus• aflatoxin• dietary fat, high blood

pressures, smoking

Evidence pyramid

Distribution: descriptive epidemiology

Person- Who?

Place- Where?

Time- When?

Guides the search to determine Why?

Assumptions in epidemiology

disease does not occur at random disease has causal and preventive factors epidemiology systematically explores

differences in disease frequency in sub-groups causal and preventive factors

Death rates by age E&W 2008

Age specific death rates

arrange people in age groupseg 15-24, 25-34, 35-44…….

count deaths in an age group count no. of people in an age group divide no. of deaths

no. of deaths

no. of people Age specific

death rate = (in an age group)

Mortality at a younger age

Using a logarithmic scale

Death rates by social class

Social class

Deathrates

Specific diseases

Deathrates

Social class

Death rates by deprivation score

Deprivation level

Deathrates

Limiting Long-Term Illness and Poor General Health, 2001

Percentage of women smoking during pregnancy, 2003

Descriptive epidemiology

person place: where time

Death rates in infants by country

Infant death rates by state

Lung cancer mortality

Colorectal cancer mortality

Stomach cancer mortality

Age-standardised death rates per 100,000 by quintile

20.6 – 41.1

41.2 – 50.7

50.8 – 57.7

57.8 – 68.9

69.0 – 136.7

Death rates from coronary heart disease in men

What does it show?

Rates of HIV diagnoses in people

What does it show?

Ischaemic Heart Disease in Men

WHO Global Atlas on cardiovascular disease, 2011

Stroke mortality in men

WHO Global Atlas on cardiovascular disease, 2011

Malaria Mortality

Endemicity of P. falciparum. Hay et al 2007

Frequency of P. vivax. Gething et al 2012

Endemicity of P. falciparum. Hay et al 2007

Prevalence of Duffy negative phenotype. Howes et al 2010

Frequency of P. vivax. Gething et al 2012

Descriptive epidemiology

• person• place• time: when

Death rates in England and Wales

Trends in CHD Mortality England and Wales

Lawlor D A et al. BMJ 2001;323:541-545

Trends in Cardiovascular Disease USA

Non-Hodgkins lymphoma 1971-2010

Monthly deaths and monthly mean temperature

Excess winter mortality by sex and age

What do the data show?

Excess winter mortality by sex and age

• Mortality goes up in winter• Affects older people more• Winter effect greater in older women• Insufficient data to comment on trends over time

Measuring disease frequency

2 main measures are usedPrevalenceIncidence

they are both rates # of diseases / # of people

Prevalence

The number of affected persons present in the population divided by the number of people in the population

# of cases (people with disease)Prevalence = -----------------------------------------

# of people in the population

Prevalence

The number of affected persons present in the population divided by the number of people in the population

If a = no. of people who have the disease b = no. of people who are disease free

aPrevalence = --------

a +b

Prevalence Example

In 1999, a US state reported an estimated 253,040 residents over 20 years of age with diabetes. The US Census Bureau estimated that the 1999 population over 20 in that state was 5,008,863.

What is the prevalence?

Prevalence ExampleIn 1999, a US state reported an estimated 253,040 residents over 20 years of age with diabetes. The US Census Bureau estimated that the 1999 population over 20 in that state was 5,008,863.

253,040Prevalence= = 0.051

5,008,863

• In 1999, the prevalence of diabetes was 5.1%(residents over 20 years of age)

• Can also be expressed as 51 cases per 1,000

Examples of prevalence

• smoking by 15 year old girls 18%• adult hypertension (150/90) 11.7%• adult schizophrenia 11 per 1000• MS in Europe 12.5 per

10,000

.

Prevalence of diabetes adults in the U.S.(Includes Gestational Diabetes)

1990 1995

2001

No Data <4% 4%-6% 6%-8% 8%-10% >10%

1999

Obesity Trends* Among U.S. AdultsBRFSS, 1990, 1999, 2008

(*BMI 30, or about 30 lbs. overweight for 5’4” person)

2008

1990

No Data <10% 10%–14% 15%–19% 20%–24% 25%–29% ≥30%

A bit more on prevalenceadult schizophrenia 11 per 1000ie at any time out of 1000 people 11 have schizophreniapoint prevalence

Suitable for conditions which are long lastingWhat about the frequency of having:

a cougha sore back

Period prevalence: measuring episodic conditions

have you had disabling back pain which lasted more than a day over the last six months six months prevalence

have you had a cough which lasted more that a day in the previous 12 months prevalence

Comparing prevalence measures:

Point prevalence 4.1%

Period prevalence (six months) 68%

Lifetime prevalence 84%

Low back pain

Prevalence

Useful for assessing the burden of disease within a population

Valuable for planning

Not useful for determining what caused disease

Check whether point or period

Incidence

The number of new cases of a disease that occur during a specified period of time

÷ the number of persons at risk of developing the disease during that period of time

# of new cases of disease over a specific period of time

Incidence = # of persons at risk of disease over that specific period of time

Lung cancer incidence in England(new cases per 100,000 per year)

Incidence of malignant melanoma (new cases per 100,000 per year)

Annual incidence

count deaths over calendar year use mid-year population as denominator

assume population size constant over the year

expressed ascases per 100,000 per yearcases per 1,000,000 per year (rare diseases)cases per 1,000 per year (common disease)

Incidence rates in sub-groups

2,000 publicans followed for 1 year48 arrests for drink drivingincidence rate = 24 per 1000 per year

ie No. of events/1000 people / year

Another example

201 adults with dementia admitted to a long-term care facility. Of the 201, 91 had a prior diagnosis of depression. Over the first year, 7 adults developed depression.

7Incidence = = 0.064

110why denominator of 110 not 201?The one year incidence of depression among adults with dementia is 6.4 per 100 or 6.4 %or 64 new depression cases per 1,000 persons with dementia

Cumulative incidence

frequency of new cases over a specified period denominator is no. of people at the start of the

period

350 people followed for 7 years and 15 cases of disease occur = 15/ 350 = 0.0429

cumulative incidence = 4.29%

Cummulative incidence of human papilloma virus after 1st intercourse

4 years, > 50%

A potential problem: lost to follow-up 350 people to be followed up for seven years say an epidemic of Black Death in second year kills 200 only 150 subsequently followed up fewer events

Cumulative incidence assumes everybody followed for same length of timeno major changes in death rates

An alternative approach

500 elderly women followed for two years say 300 died exactly at end of first year then 300 for 1 year = 300 person years at risk 200 for 2 year = 400 person years at risk total = 700 person years at risk

73 had hip fracturesincidence rate = 73 / 700 =0.104

or = 10.4 per 100 per year

Allowing for lost to follow up/death

Ten people aged 90 years followed up for 5 yearsnone died in first year

2 died in second year

1 died in third year

5 died in fourth year

none died in fifth year

How many person years at risk?assume deaths occurred at mid year

What was the incidence rate?

Adding it up

2 people lived 1.5 years= 3

1 person lived 2.5 years = 2.5

5 people lived 3.5 years= 17.5

2 people lived 5 years = 10 years

Total = 33 person years

Person years mortality = 8/ 33

= 0.24

= 24 deaths per 100 person years

Different types of incidence rate

annual incidence from routine data

cumulative incidence events divided by initial population

person years incidence allows for loss to follow up

What (annual) incidence tells us

number of new casesover defined period

sensitive to changes in disease risk more suitable for monitoring trends less suitable for assessing burden of disease

Incidence is different to prevalence

incidence number of new cases (per 1000)

• in a defined period

prevalence number of existing cases (per 1000)

Exploring prevalence: an initial look

Prevalence

= prevalent cases

Some time later

Old (baseline) prevalence

= prevalent cases = incident cases

New prevalence

Incidence

No cases die or recover

Later still

= prevalent cases = incident cases = deaths or recoveries

An example to work out

A town has a population of 3600. In 2003, 400 residents of the town are diagnosed as having a disease. The disease is lifelong but it is not fatal.

In 2004, 200 additional residents of the town are diagnosed with the same disease.

•What is the prevalence in 2003? In 2004?•What is the incidence in 2004?

Answers

• Population : 3600• 2003: 400 diagnosed with a disease• 2004: 200 additional diagnosed with the disease• No death, no recovery

Numerator

Denominator

Prevalence (2003)

400

3600

11.1%

Prevalence (2004)

600

3600

16.7%

Incidence (2004)

200

3200

6.3%

Prevalence and incidence of tuberculosis

Prevalence (per 1000)

Incidence (per 1000 per year)

Poorville 60 20Richville 70 10

Poorville: survival time = 3 years

Year Existing cases

New cases

Deaths Prevalence*

1 0 20 0 202 20 20 0 403 40 20 0 604 60 20 20 605 60 20 20 606 60 20 20 60

* survey on 31st December

Richville: survival time = 7 yearsYear Existing

casesNew cases

Deaths Prevalence*

1 0 10 0 102 10 10 0 203 20 10 0 304 30 10 0 405 40 10 0 506 50 10 0 607 60 10 0 708 70 10 10 709 70 10 10 70

* survey on 31st December

Prevalence and Incidence

Prevalence depends on the annual incidence of disease and the duration of disease (in years)

Prevalence and incidence

Incidence (per 100,000 per

year)

Duration (years)

Prevalence (per 100,000)

Poorville 20 3 60

Richville 10 7 70

Prevalence = Incidence x average disease duration(If incidence, survival and cure rates are constant)

Prevalence and incidence

Prevalence = Incidence x average disease duration(If incidence, survival and cure rates are constant)

Incidence (per 100,000 per year)

Duration (years) Prevalence (per 100,000)

Lung cancer 80 0.4 32

Breast cancer 20 5 100

Depression 12,000 0.5 6000

Common cold 50,000 0.01 5,000

Things you should know definition of epidemiology descriptive epidemiology

person, place, time: clues to possible causes

prevalencepointperiod

incidenceannual cumulativeperson years

P = I x D