Public Health and Epidemiology

Question 1

Absolute Risk Reduction
DIFFERENCE in the RATE of adverse events in exposed from unexposed groups

ARR is mathematically equivalent to attributable risk ATTRIBUTABLE RISK for risk factors

e.g.
Risk of DVT falls from 0.03 to 0.02% after stopping CHC
therefore the absolute risk reduction is 0.01% or 1 in 10000

Note: to convert % into ratio
Example 0.02/100 ->

0.02/0.02 = 1
100/0.02 = 5000

Answer 1

Accuracy:
How close measured value is to standard or known value

NB: note difference between accuracy and PRECISION
e.g. 10 BP readings by 10 different doctors: little variation, so result is PRECISE

The BP machine is not calibrated so the result is INACCURATE

Precision : closeness of 2 or more measurements (independent of accuracy)

Question 2

ATTACK RATE:
Proportion of individuals who are at risk of an infectious disease -> who become clinically ill in a given time.

Usually a percentage:

Example :
In an outbreak of food poisoning, from 1000 diners, 50 cases of diarrhoea within one week of the meal.

Attack rate : 50/1000 = 5%

Answer 2

Attributable Fraction:
Proportion of an outcome in people ENTIRELY due to A RISK FACTOR

AR/risk in the exposed population

Example :smokers have a 25.2% lifetime risk of lung cancer. non smokers have a 0.2% lifetime risk

AR = 25.2.% - 0.2%

AF = 0.2%/25.2 = 0.99 = 99%

99 out of 100 lung cancers IN SMOKERS due to smoking

Attributable Risk - difference in RATE of disease in exposed group compared to unexposed.

• identical to absolute risk reduction for the EFFECT of treatment

Question 3

Basic Reproductive Number (R0)
AVERAGE number of individuals directly infected by an infectious case, during ENTIRE infectious period when entering totally susceptible population

e.g.
R0= 12 for measles
each case of measles introduced in non immune population would result in infection in 12 other people

Answer 3

Clinically significant:
An effect that is large enough that is it worth treating the patient

Statistically significant:
An effect unlikely to have occurred by random chance

Question 4

Confidence Interval:
a calculated interval within a given probability (95%)
the TRUE VALUE OF EFFECT lies in this interval
5% risk the true value lies outside this interval

P value:
Probability an affect AT LEAST AS EXTREME as that observed in the study could have occurred by chance alone.

This is NOT the probability of making a Type 1 error i.e. falsely rejecting a null hypothesis)

Answer 4

Confounding variable:
Factor independently associated with exposure and outcome/disease
BUT NOT THE INTERMEDIATE FACTOR between exposure and outcome

Question 5

Cost - minimisation
Compare alternative interventions according to cost. OUTCOME is the SAME

Cost utility analysis 
 - formal comparison 
 - costs and consequences not equal
 - results in 'costs per unit of outcome' 
 - commonly CUA measured with QUALITY  
 ADJUSTED LIFE YEARS

3. Quality Adjusted Life Year
   - incorporates quantity and quality dimensions into a measurement
   - calculation :
   
   1. Total life years gained
   2. Weighting each year with QoL (0-1)

E.g.
12 month prognosis with QoL score of 0.4
After drug:
Increase of 12 months and QoL 0.8

(2years x 0.8) - (1year x 0.4) = 1.2 QALYs gained

Answer 5

Effective Reproductive Number:
Directly infected by infectious case…

During entire infectious period..

When entering a typical population of susceptible and non susceptible individuals

For example :
Each case of measles R0 = 12 introduced in a population, half of whom were immune to measures, would result in 6 people developing infection

Question 6

Endemic : maintenance of disease or illness in a region without external inputs

E.g.
HSV in endemic in UK
It does not need an external increase in cases.
Malaria by comparison is not maintained due to the lack of vector

Epidemic: occurrence of disease or illness in EXCESS OF NORMAL EXPECTANCY

E.g.g flu epidemic is declared when case numbers exceed that which is usually expected

Pandemic : epidemic over wide area or worldwide - crossing international boundaries

E.g. Viral haemorrhaging fever pandemic declared when more case than expected, in different countries and parts of the world

Answer 6

Evidence Based Medicine:
Ability to ACCESS, SUMMARISE and APPLY

.. information from literature

.. to day to day clinical practice

Question 7

False Negative : test result negative but person does have disease

False Positive : test result positive but person does NOT have disease

Answer 7

GDP: Measures total output (goods and services) within a geographical boundary, regardless of the nationality of entities producing the output

PPP: to compare living standards across countries

… exchange rates are constructed

… comparing national prices for a large basket of goods and services

These rates are used to convert different currencies into a COMMON CURRENCY to measure purchasing power of per capita income in different countries

Example :
USD will buy you a lot more in India than America
- GDP measured as PPP per capita in India is $5140 compared to $1490 gross GDP per capita
- GDP per capita as PPP of USA is $50000

Opportunity cost : the total lost benefit from resourcing the next best alternative

Question 8

Herd Immunity Threshold : Proportion of population who need to be immune for TRANSMISSION to decline

E.g. Once 95% children immunised, measles not longer has a large enough susceptible population to sustain replication, and case numbers will decline

Incubation Period: time interval between acquisition of infectious agent, and first sign or symptom of disease

E.g. It takes 7-12 days for symptoms of measles to appear after exposure

Infectious Period: time interval when transmission to susceptible host possible

Latent Period :time interval between acquisition of infectious agent and onset of infectiousness

E.g. Measles rash 7-12 d from acquisition
Infectious from 4 days before rash
THEREFORE latent period 3 - 8 days

Answer 8

Pre test Probability: that patient has disease before any investigations.

Equal to prevalence of disease in people with same clinical features as INDEX patient

E.g. 20% men over 65 have prostate cancer
Pre test probability of prostate cancer is 20%

Post test probability:
Probability that patient has disease AFTER test carried out

This is EQUAL to POSITIVE PREDICTIVE VALUE

E.g.
Following PSA test in above example, 30% with positive test have disease, and 70 % who tested positive do not have disease.

Post test probability is 30% (having disease with positive test)

Question 9

INCIDENCE : number of NEW cases or events in a specified time period

e.g. in 2011, lung cancer incidence among males in UK was 77 NEW cases per 100,000

PREVALENCE : proportion of individuals in population with a disease or condition at a given time

e.g. Prevalence of lung cancer in the UK was 85 per 100,000 in 2006

Answer 9

RATE =

a. frequency at which events occur
b. in a defined timeframe
c. often expressed as number of event PER total person-‘time’ at risk of developing disease

e.g.
rate of lung cancer in 2006 among new smokers was 15 cases per 100000 person-years

RATE RATIO +

a. ratio of ratio of disease in exposed group
b. to rate of disease in unexposed group

> 1 = rate is greater in exposed group

= : no difference in rates between exposed and unexposed group

<1 = rate is lower in exposed group