Lecture 2. Types of Study Design

Question 1

What was the research into scurvy in sailors a good example of?

Answer 1

One of the earliest and best recorded experimental studies

Question 2

How are epidemiological studies split?

Answer 2

Between experimental and observational study designs

Question 3

What is a type of an experimental study design?

Answer 3

Randomised control trial (RCT)

Question 4

Give four types of analytical observational study designs?

Answer 4

Cohort (longitudanal)
Case-control
Cross-sectional
Ecological (population-based)

Question 5

What is an ecological/population-based study and why isn’t it used as often?

Answer 5

A study design where the unit of inference is a group
Plotting proportions based on aggregation
Subject to the most bias

Question 6

What would be an example of an experimental study?

Answer 6

Clinical trail for a new antibiotic or impact of putting fluoride in the water

Question 7

What are the requirements for a study to be experimental?

Answer 7

Observational units are randomly assigned to one of two levels of an exposure
All other exposures are identical between the two groups (controlled)
Subjects are as similar as possible or stratified
They maximise a researcher’s ability to observe one effect independent of other variables

Question 8

What would be an example of an observational study?

Answer 8

Natural exposures: compare disease in smokers vs. nonsmokers?
Historical controls: Compare obesity rates over time?

Question 9

What makes observational studies different from experimental studies?

Answer 9

Data are collected without randomisation or artificial manipulation of predictor variables
More difficult to identify causes because it is hard to tease out individual variable effects
There are fewer ethical constraints on the research

Question 10

How are observational studies split?

Answer 10

Between being descriptive or analytical

Question 11

What are examples of descriptive observational studies (estimation)?

Answer 11

Proportion of women with breast cancer (prevalence)
The rate of cases of measles in one year (incidence)
Case studies describing a new disease or treatment

Question 12

What do descriptive observational studies describe but not identify?

Answer 12

Describe what is occurring and typically quantify a variable
Do not identify causes
Used to estimate disease outbreaks, but don’s say why they occur

Question 13

What are analytical or testing observational studies?

Answer 13

Studies where statistical comparisons are made to identify associations
Can allow causal inference

Question 14

What are examples of analytical studies?

Answer 14

Comparing the symptoms and progression of Parkinson’s in patients taking a new vs old drug
Comparing pesticide concentrations (chlordecone, an endocrine disruptor) in individuals with and without prostate cancer

Question 15

What is required for all analytical studies?

Answer 15

Identification of variables that are associated with a disease by comparing groups
Interpretation: Is the association ‘real’ or due to confounding?, Is the association causal? Risk factors are real associations that are not necessarily casuals
Consideration of the risk of bias and confounding factors throughout the investigation

Question 16

What are the key features of a randomised control trial?

Answer 16

Explicit randomisation of treatment or exposure
Defined target population
Following patients forward in time to see if they improve or not

Question 17

What is an example of an experimental study?

Answer 17

How do intestinal nematodes (and their treatment) influence susceptibility to TB, mortality from TB and its spread within African buffalo?

Question 18

When are experimental studies carried out?

Answer 18

After we are fairly sure what is causing the differences that we have found in comparative, analytical studies

Question 19

What does experimental design allow us to conclude?

Answer 19

Differences in the dependent variable are due to differences in the treatment

Question 20

What happens in experiments studies that do not allow for random allocation of treatments?

Answer 20

Remains subject to bias

Question 21

What are cohort studies?

Answer 21

Follow groups of individuals through time until the exposure and outcome happens in a defined proportion of the sample

Question 22

How are cohort studies different from randomised control trials?

Answer 22

Cohort studies have a non-random assignment of exposures due to ethical reasons

Question 23

What is an example of a cohort study?

Answer 23

Framingham Study (1948)
Study population: Age 30-62 in Framingham
Hyp: Incidence of Coronary Heart Disease (CHD) increases with age and is higher in men
Hyp: High cholesterol, weight, tobacco use is associated with increased risk of CHD

Question 24

What are the properties of a cohort study?

Answer 24

Good at identifying causes because the change occurs after the observations (time sequence)
Data on exposures are gathered as they happen (no recall bias)
Expensive and time consuming - “loss to follow-up” (people might say they don’t want to be in the study anymore)
Difficult to analyse (repeated observations - info is not independent, it is reliant on using the same people every time)

Question 25

What are case control studies?

Answer 25

Take individual with disease (cases) and compare them with individuals from the same population without the disease (controls) Calculate proportion of cases that were exposed & proportion of controls that were exposed

Question 26

What is an example of a case control study?

Answer 26

Breast cancer in women Select cases - women with breast cancer Select controls - women from the same referral unit with another non-linked disease, the same GP practice list, the same town Compare the diet, lifestyle, life experiences of the two groups Match on age, race, social group

Question 27

What are the properties of case control studies?

Answer 27

Looks backwards (vs. cohort studies look forwards) Begins with identifying cases (vs. cohort study identifies exposure groups) More difficult to identify causes compared to cohort study but better at identifying causes than cross-sectional studies because you can still assess the temporal sequence of risk vs. disease Are cheap and quick to do They rely heavily on recall or past records, which may be inaccurate or bias Rely on selection of cases

Question 28

What are cross sectional studies?

Answer 28

Predictor and response variables are measured at one point in time

Question 29

What are examples of cross sectional studies?

Answer 29

Questionnaire designed to evaluate the factors associated with alcoholism that also asks about lifestyle and feelings Randomly sampled African buffalo populations designed to evaluate if co-infection influenced nematode burdens

Question 30

What is the big problem with cross-sectional studies?

Answer 30

Which came first? They demonstrate association but cannot attribute cause Did alcoholism lead to lifestyle changes of vice versa? Did the coccidia influence susceptibility to nematodes or vice versa? Did a confounding factor influence them both?

Question 31

What are the properties of cross-sectional studies

Answer 31

Poor at identifying causes (vs. case-control and cohort studies that consider timing) Good at generating hypotheses (good to do when there is little known about a subject) Quick and relatively cheap Consequently, the literature and media abound with ad hoc cross-sectional surveys