Epidemiology

Question 1

what is epidemiology

Answer 1

The study of how often diseases occur in different groups of people and why.

Epidemiological information is used to plan and evaluate strategies to prevent illness and as a guide to the management of patients in whom the disease has already developed.

Question 2

What is evidence based medicine

Answer 2

The process of systematically reviewing, appraising and using clinical research findings to aid the delivery of optimum clinical care to patients

Question 3

Evidence based medicine needs evidence that is….

Answer 3

reliable and relevant

Question 4

Cause/treatments for disease is answered by what type of studies

Answer 4

• Cause is answered by observational studies
• Treatments are evaluated by intervention studies

Question 5

What was the main aim of Doll and Bradford Hills study

Answer 5

They wanted to try and find the cause of a sharp rise in lung cancer cases

Question 6

How did Doll and Bradford Hill conduct their study

Answer 6

• Interviewed 700 lung cancer patients and 700 non-cancer patients
• both groups similar ages and matched proportions of males to females
• Smoker = one or more cigarettes a day for at least previous year
• non smoker - had less
• They then tabulated their patients by lung cancer vs. other disease and smoker vs. non-smoker

Question 7

What was the results from Doll and Bradford Hills study

Answer 7

The proportion of smokers in lung cancer group is much higher than in the other disease group.

Suggests there is an association but doesn’t prove alone that smoking causes lung cancer.

Question 8

How do you obtain a sample for an epidemiological study

Answer 8

1) It needs to be based on a well defined population (e.g. females under 20 or males between 20-55 living within 10 miles of city centre)

2) An epidemiological study then takes a sample from that population

3) However limits the extent to how applicable it is to the rest of the population outside of the well defined population from step 1.

Question 9

What does the sample need to be in order to be valid

Answer 9

representative of the population

Question 10

How does epidemiological studies classify people with or without disease

Answer 10

if you have a disease you are a case, if you don’t you are normal

Question 11

cases can be defined in what three ways

Answer 11

• Statistical
• Clinical
• Operational

Question 12

statistical definition of a case

Answer 12

Unusual people though not necessarily diseased
e.g. IOP has a mean of (16mmHg) and standard deviation of 4. Anyone who has IOP of 2 SD above the mean (24mmHg) defined as ocular hypertension case.

Question 13

Clinical definition of a case

Answer 13

If studying visual acuity, we define a case as someone who has suffered a reduction in their visual acuity

Question 14

Operational definition of a case

Answer 14

if studying myopia we may divide people into low myopes or high myopes.
this is an operational definition of high myopia
diving line is arbitrary e,g, -5D or more

Question 15

What is the difference between observational studies and intervention studies

Answer 15

Observational - those which just measure level of disease or condition, identify factors regarded
as causal in the development of a disease (risk factors), etc.

Intervention studies - those which test a new treatment to see if it is effective

Question 16

which studies extend into the past, present or future

Answer 16

observational studies –>
- cross sectional = present
- cohort = present and future
- retrospective cohort = present and past
- case control = present and past

intervention studies –>
clinical trial = present and future

Question 17

What is a cross sectional study

Answer 17

• It looks at health as a specific moment in time
• it can find associations between risk factors and disease
• it can suggest but not prove that a risk factor causes a disease
• it cannot address temporal relationships (don’t know which disease came first A or B)

Question 18

what does prevalence mean

Answer 18

It is the fraction of the study population who are cases at a specific moment

number of cases/ sample size

Question 19

what is crude prevalence and specific prevalence

Answer 19

• Crude prevalence is the number of cases divided by the size of the population (everyone no matter age)
• Specific prevalence is the prevalence in a subpopulation (different groups of different ages)

Question 20

what is a prevalence ratio

Answer 20

the ratio of two prevalences says how much more/less common a disease is in one group than another

For example, the cataract prevalence ratio for age 85-100 compared to age 80-84 is
59% / 48% = 1.23
- This means you are 1.23 times more likely to have cataract if over 85 than if between 80 and 85
- alternatively there is 23% more cataract in the older age group.

Question 21

odds ratio

Answer 21

• used to measure the strength of an association
• If you say the odds of winning are 2-to-1, you mean:
  } There are 2 “chances” of winning
  } And one “chance” of losing
  } So three “chances” in total
  } So the probability of winning is 2/(2+1) = 2/3

Question 22

prevalence and odds

Answer 22

to work out prevalence its the number of ppl with cataracts over the total number of people in that group

to work out odds its the number of people with cataracts divided by the number of people without cataracts.

Question 23

the odds ratio is then…

Answer 23

a ratio of the two odds
so odds ratio is 85-100: 80-84
1.43/0.925 = 1.55
prevalence ratio 85-100: 80-84
59/48 = 1.23

Question 24

why use odds ratio

Answer 24

• It contains the same information as the prevalence ratio.
• In some study types, you can’t calculate prevalence ratios, but you can calculate odds ratios
• When the disease is rare, the odds ratio and the
  prevalence ratio are similar.
• The odds ratio can be used as a measure of association (a bit like a correlation).

Question 25

interpreting odd ratio

Answer 25

• if no association, odd ratio close to one
• if positive association between risk factor and disease, odd ratio is greater than one (means risk factor increases chance of disease)
• if negative association between risk factor and disease, odds ratio is less than one (mean risk factor decreases chance of having the disease)

Question 26

Cohen’s rule of thumb

Answer 26

positive association:
between 1 and 1.45 = small or no association
around 3.5 = medium association
around 10 = large/strong association

negative association:
between 0.69 and 1 = small or no association
around 0.3 = medium association
around 0.1 = large/ strong association