Epidemiology

Question 1

What is the definition of epidemiology?

Answer 1

The study of the frequency, distribution and determinants of diseases and health-related states in populations in order toe prevent and control disease

Question 2

What is the incidence?

Answer 2

Number of new cases; the denominator is the number of disease free people at the start of the study; it is over time

Question 3

What is the prevalence?

Answer 3

Number of existing cases; it is at a point in time

Question 4

What is person-time?

Answer 4

Person-time is a measure of time at risk. i.e. time from entry to a study to (i) disease onset, (ii) loss to follow-up. (iii) end of study

Question 5

What is person-time used for?

Answer 5

It is used to calculate incidence rate which uses person-time as the denominator. (cumulative incidence uses the number of disease free people at the beginning of the study)

Question 6

What is incidence rate used for?

Answer 6

It is useful when study participants are followed up for varying lengths of time

Question 7

What is the incidence rate?

Answer 7

Number of persons who have become cases in a given time period / Total person-time risk during that period

Question 8

What are the usual headings when describing the epidemiology of a disease?

Answer 8

Time; Place; Person (age, gender, class, ethnicity)

Question 9

What is absolute risk?

Answer 9

It has actual numbers and units, so per number of people

Question 10

What is relative risk?

Answer 10

Risk in one category relative to another

Question 11

What is the attributable risk?

Answer 11

The rate of disease in the exposed that is due to the exposure

Question 12

What does the relative risk tell us about?

Answer 12

About the strength of association between a risk factor and a disease

Question 13

What does the attributable risk tell us about?

Answer 13

About the size of effect in absolute terms

Question 14

What is bias?

Answer 14

A systematic deviation from the true estimation of the association between exposure and outcome

Question 15

What are the two main types of bias?

Answer 15

Selection bias and information bias

Question 16

What is a systematic bias?

Answer 16

An error in the selection of study participants or the allocation of participants into different study groups

Question 17

What is an information bias?

Answer 17

A systematic error in the measurement or classification of exposure or outcome.

Question 18

What are the sources of information bias?

Answer 18

Observer (observer bias); participant (recall bias); instrument (wrongly calibrated instrument)

Question 19

What is confounding?

Answer 19

The situation where a factor is associated with the exposure of interest and independently influences the outcome (but does not lie on the causal pathway)

Question 20

What should you consider when looking at association and causation?

Answer 20

Bias, chance, con-founders, criteria

Question 21

What are the factors when assessing causality?

Answer 21

Strength of association; Dose-response; Consistency; Temporality (does exposure come before outcome); Reversibility (if you take away the exposure the outcome goes away); Biological plausability

Question 22

How can you classify prevention?

Answer 22

Primary, secondary and tertiary

Question 23

What is primary, secondary ad tertiary prevention?

Answer 23

Primary - no disease
Secondary - pre-clinical disease
Tertiary - clinical disease

Question 24

What is the population approach to disease prevention?

Answer 24

A preventative measure delivered on a population wide basis and seeks to shift the risk factor distribution curve

Question 25

What is the high risk approach to disease prevention?

Answer 25

It seeks to identify individuals above a chosen cut-off and treat them

Question 26

What is the prevention paradox?

Answer 26

A preventive measure which brings much benefit to the population often offers little to each participating individual.

Question 27

What is screening?

Answer 27

A process which sorts out apparently well people who probably have a disease (or precursors or susceptibility to a disease) from those who probably do not

Question 28

What are the types of screening?

Answer 28

Population-based screening programmes
􏰀Opportunistic screening
Screening for communicable diseases
􏰀Pre-employment and occupational medicals
􏰀Commercially provided screening

Question 29

What is sensitivity?

Answer 29

The proportion of people with the disease who are correctly identified by the screening test

Question 30

What is specificity?

Answer 30

The proportion of people without the disease who are

correctly excluded by the screening test

Question 31

What is the positive predictive value?

Answer 31

The proportion of people with a positive test

result who actually have the disease

Question 32

What is the negative predictive value?

Answer 32

The proportion of people with a negative

test result who do not have the disease

Question 33

Why are positive predictive values so different?

Answer 33

Predictive values are dependent on underlying prevalence

Question 34

What is lead time bias?

Answer 34

The length of time between the detection of a disease (usually based on new, experimental criteria) and its usual clinical presentation and diagnosis (based on traditional criteria).

Question 35

What is length time bias?

Answer 35

Length time bias is a form of selection bias, a statistical distortion of results that can lead to incorrect conclusions about the data. Length time bias can occur when the lengths of intervals are analysed by selecting intervals that occupy randomly chosen points in time or space. That process favors longer intervals and so skews the data.