epidemiology

Question 1

what are measures of occurrence

Answer 1

Describe the distribution of an outcome in a group of people in regards to Person, Time and Place:
* Prevalence
* Incidence: Cumulative incidence
(risk), incidence rate (rate) * Proportions vs odds

Question 2

what are measures of association

Answer 2

Describe association between outcome and its determinants (exposures, risk factors, interventions):
* Ratios
* Differences
* Regression coefficient

Question 3

Point prevalence

Answer 3

measures the proportion of existing people with a disease in a defined population at a single point in time.

no with disease/ total of population at specific time point

Question 4

Period prevalence

Answer 4

is the number of individuals identified as cases during a specified period of time/ the total number of people in that population.

Question 5

cumulative incidence (RISK)

Answer 5

number of new cases in specific time period/ number of people at risk at start of time period

(proportion)

Question 6

incidence rate (RATE)

Answer 6

number of new cases in specific time period/ total person-time at risk during time period

Question 7

odds (of a disease)

Answer 7

number of new cases/ number who dont have disease at end of period

(part/none part)

Question 8

if there is an association what could be the cause?

Answer 8

• chance
• bias
• confounding
• true association
• reverse causation

Question 9

What are ratio/relative measures of association?

Answer 9

Divide the two measures of disease occurrence

CIR, IRR, OR

How much higher is the risk of lung cancer among smokers (exposed) compared to non smokers (unexposed)?

Question 10

cumulative indicidence ratio

Answer 10

CI in exposed/ CI in unexposed

Question 11

incidence rate ratio

Answer 11

incidence rate in exposed/ incidence rate in unexposed

Question 12

what do these relative risk mean.

<1

1

> 1

Answer 12

A relative risk of 1 indicates that the incidence of disease in the exposed and unexposed groups is identical and that there is no association observed between the disease and risk factor/ exposure.

A relative risk > 1 occurs when the risk of disease is greater among those exposed and indicates a positive association, or an increased risk among those exposed to the risk factor compared with those unexposed.

A relative risk < 1 occurs when the risk of disease is lower in those exposed compared to those unexposed and indicates a negative association.

Question 13

Attributable risk

is it absolute or relative

Answer 13

risk in exposed - risk in unexposed

absolute

Question 14

what do these mean for the attributable risk

<0

0

> 0

Answer 14

<0 exposure associated with X fewer cases

= 0 no association between exposure and disease

> 0 exposure associated with X more cases compared to unexposed

Question 15

Attributable Risk Fraction Aetiological Fraction Etiological Fraction

Answer 15

R exposed - R unexposed/ R exposed

Question 16

population attributable fraction

Answer 16

Risk in total population - risk in unexposed

The population attributable risk (PAR) estimates the excess rate of disease in the total study population that is attributable to the exposure. It provides a measure of the public health impact of the exposure in population,

assumes that the association is causal!!!

Question 17

Number Needed to Treat (NNT)

Answer 17

1/ absolute risk reduction (ignoring sign)

number of people needed to treat to prevent 1 case

look at absolute risk reduction to see if worthwhile

Question 18

methods of probability sampling

Answer 18

random
stratified
systematic
cluster

Question 19

what is person years definition, What are the advantages and disadvantages.

Answer 19

Definition
Patient years=denominator of the rate, calculated by counting all individuals in a pre-determined time period in this case time spent in the RCT

Advantages
Using person-time rather than just time handles situations where the amount of observation time differs between people,

Can be used to compare between studies as a method of standardisation (e.g. comparing studies of different lengths) and can be used to maximise all data as patients will have been in RCT for variable length of time

Disadvantage
Use of this measure implies that the incidence rate is constant over different periods of time, such that for an incidence rate of 14 per 1000 persons-years, 14 cases would be expected for 1000 persons observed for 1 year or 50 persons observed for 20 years. This may not be the case.
Difficult to interpret and communicate in the real world as requires understanding of the assumptions underlying it

Question 20

Describe an appropriate method for comparing the two survival curves, what assumptions is this test based on

Answer 20

The log rank test.
null: no difference between survival curves

(used on kaplan meier curves)

Assumptions
1) that the data are continuous or ordinal

2) that risk of an event in one group relative to the other does not change over time (proportional hazards assumption)

3. non parametric

Question 21

what is censoring? give some reasons

Answer 21

Censoring is when an observation is incomplete due to a cause independent of the event of interest

the subject dies from another cause, independently of
the cause of interest;
the study ends while the subject survives; or
the subject is lost to the study, by dropping out, moving
to a different area
Event of interest already occurred at the observation
time, but it is not known exactly when.

Question 22

Relative Risk Reduction (RRR)

Answer 22

= (ARc – ARt) / ARc; or RRR = 1 – RR

c: control
t: treatment
RR: relative risk - ARt/ ARc

Question 23

proportional mortality ratio

Answer 23

measures the proportion of deaths occurring from a given
cause for a particular occupation relative to the proportion of deaths from that cause in the
whole population (or a comparison population)

 A PMR is based on the underlying cause of death as recorded on the death certificate and provides little information about diseases that cause high morbidity but which are
rarely fatal

 Occupational information usually relates to the deceased persons last full-time job, but other jobs undertaken earlier in their career may be of more relevance
 The PMR is not a population-based rate, and so it can be used to measure mortality where no denominator is available about the number of people who work in an
occupation
 The PMR of an occupational group for a specific cause depends not only on its death rate from the cause in question, but also on its death rate from all causes combined.
This may be of relevance where an occupational group has particularly high or low mortality from the most common causes of death. As it is a proportional measure, the
PMR can be raised in situations where the frequency of other causes of death is low

Question 24

Relative Risk (RR)

Answer 24

= ARt / ARc (AR exposed/ ER unexposed)

Question 25

what type of graph is this and give four things that it shows

Answer 25

Epidemic curve

• duration of outbreak
• magnitude of outbreak
• time trend of outbreak
• propagation of outbreak (person to person spread)
• outlier investigation

Question 26

basic reproduction number and what is it affected by

Answer 26

average number of secondary infections produced by a typical case of an infection in a population where everyone is susceptible

affected by:
The rate of contacts in the host population (eg behaviours)
The probability of infection being transmitted during contact
The duration of infectiousness.
how infection spread (direct contact, airborne)

Question 27

effective reproductive number

Answer 27

average number of secondary cases caused by a disease in a particular population, at a particular time.

OR

average number of secondary cases per infectious case in a population made up of both susceptible and non-susceptible hosts.

Question 28

herd immunity threshold

Answer 28

the proportion of a population that need to be immune in order for an infectious disease to become stable in that community. If this is reached, for example through immunisation, then each case leads to a single new case (R=1) and the infection will become stable within the population.

(R0-1) /Ro

If the threshold for herd immunity is surpassed, then R<1 and the number of cases of infection decreases. This is an important measure used in infectious disease control and immunisation and eradication programmes.

Question 29

Attack rate

Answer 29

total number of new cases/ population at risk in a defined time period

Question 30

how to calculate excess deaths

Answer 30

1. define baseline risk (lowest exposure)
2. ensure each group has a death rate
3. death rate A - baseline death rate = excess death rate
4. excess death rate x population n A = excess deaths

Question 31

Life table (how to calculate)

Answer 31

1. calculate deaths, survivors
2. work out life years lived for each age band = survivors + (deaths/2)
3. sum all life years
4. life expectancy = sum life years/ alive at start

for life year (Lx)

times by age band

Question 32

cohort vs period life expectancy

Answer 32

Cohort:
- the probability of a person from a given cohort dying at each age over the course of their lifetime. a cohort refers to a group of people with the same year of birth.

• The cohort life table is based on age-specific probabilities of death, which are calculated using observed deaths (mortality) data from the cohort.

-A cohort life table uses a combination of OBSERVED AND PROJECTED mortality rates

good: more accurate, can project

issues: how projections calculated, complicated to produce

Period:

• mortality rates for a fixed period in time. (no projections)

For example, period life expectancy at age 65 years in 2020 would use the mortality rates for 2020 for ages 65, 66 and 67 years, and so on.

use mortality rates from a single year (or group of years) and assume that those rates apply throughout the remainder of a person’s life. This means that any future changes to mortality rates would not be taken into account

good: baseline for cohort LE

issues: normally more conservative LE