Epidemiology Flashcards
what is epidemiology
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.
What is evidence based medicine
The process of systematically reviewing, appraising and using clinical research findings to aid the delivery of optimum clinical care to patients
Evidence based medicine needs evidence that is….
reliable and relevant
Cause/treatments for disease is answered by what type of studies
- Cause is answered by observational studies
- Treatments are evaluated by intervention studies
What was the main aim of Doll and Bradford Hills study
They wanted to try and find the cause of a sharp rise in lung cancer cases
How did Doll and Bradford Hill conduct their study
- 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
What was the results from Doll and Bradford Hills study
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.
How do you obtain a sample for an epidemiological study
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.
What does the sample need to be in order to be valid
representative of the population
How does epidemiological studies classify people with or without disease
if you have a disease you are a case, if you don’t you are normal
cases can be defined in what three ways
- Statistical
- Clinical
- Operational
statistical definition of a case
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.
Clinical definition of a case
If studying visual acuity, we define a case as someone who has suffered a reduction in their visual acuity
Operational definition of a case
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
What is the difference between observational studies and intervention studies
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
which studies extend into the past, present or future
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
What is a cross sectional study
- 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)
what does prevalence mean
It is the fraction of the study population who are cases at a specific moment
number of cases/ sample size
what is crude prevalence and specific prevalence
- 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)
what is a prevalence ratio
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.
odds ratio
- 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
prevalence and odds
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.
the odds ratio is then…
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
why use odds ratio
- 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).
interpreting odd ratio
- 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)
Cohen’s rule of thumb
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
