Study Designs

Question 1

What is Epidemiology?

Answer 1

The study of human populations. It studies how often diseases occur in different groups of people and why.

Question 2

What does Nutrition Epidemiology investigate?

Answer 2

The relationship between dietary intake and disease development. Results are often used to explore the relationship between nutrition and health outcomes.

Question 3

What are the 2 types of Study Design?

Answer 3

Experimental and Observational

Question 4

What are the characteristics of the Observational Studies (surveys)?

Answer 4

Association, not causation, limited to responders, data collected/questions, rely on self-report (diet)

Question 5

What are the characteristics of the Intervention Studies (treatment vs control)?

Answer 5

Better, but not always possible/ethical, limited time effect, limited to group characteristics, controlled environment (lab)

Question 6

What are the 3 types of observational studies?

Answer 6

Cross-sectional (present)
Case-control (past-present)
Prospective cohort (present-future)

Question 7

What is the Hierarchy of Scientific Evidence (from weakest to strongest)?

Answer 7

1. Case reports, opinion papers and letters
2. Animal trials and in vitro studies
3. Cross-sectional studies
4. Case-control studies
5. Cohort studies
6. Randomized controlled trials
7. Meta-analysis and systematic reviews

Question 8

What are the characteristics of cross-sectional studies?

Answer 8

They can be used to describe the prevalence of nutrition problems in a population.
Information on population characteristics can help assess whether these factors impact the exposure-outcome relationship.
When repeated in the same population, a cross-sectional survey can be used for surveillance and monitoring.
A cross-sectional study led to a phenomenon known as the “French Paradox”.

Question 9

How can sampling affect cross-sectional studies?

Answer 9

Selection bias and sample size will affect the generalizability of the findings.
If a particular sector of the sample is excluded, the prevalence estimate and reported associations may be misleading.
If the sampling frame is adequate, but the sample size is too small to provide a reliable estimate of the population prevalence, the study results will not be helpful.

Question 10

What are the limitations of cross-sectional studies?

Answer 10

They are considered the weakest type of epidemiology because they are based on group outcomes.
It may lead people to believe that members of the group have characteristics which, as individuals, they do not. This is known as the ecologic fallacy.
Results must be interpreted with caution.
Difficult to establish what is cause and what is effect.
Causal inferences cannot be drawn, because the sequence of exposure/outcome cannot be established.

Question 11

Example of limitations of cross-sectional studies

Answer 11

If milk drinking is associated with peptic ulcer, is it because milk causes the disease, or is it because ulcer sufferers drink milk to relieve their symptoms?

Question 12

How are case-control studies used?

Answer 12

To determine the degree of association between various risk factors and outcomes

Question 13

Which groups do case-control studies look at?

Answer 13

Cases (people with a disease, condition or disability)
Controls (people without the disease)

Question 14

What are the characteristics of case-control studies?

Answer 14

Most case-control studies are retrospective.
Patients with a disease (Cases) are compared with controls who do not have the disease.
The study begins by recruiting people based on their outcome status (disease) and then explores past exposure measures.
Cases and controls are matched on characteristics such as age, sex, BMI, etc.
Exposure levels between the cases and controls are then compared.

Question 15

What are the strengths of case-control studies?

Answer 15

Inexpensive
Efficient
Not time-consuming in comparison to other study designs
Suitable for rare diseases with long latency periods.

Question 16

What are the limitations of case-control studies?

Answer 16

Due to their retrospective nature, case-control studies are subject to recall bias.
Selection bias (some people might be excluded due to their location, attendance of a particular clinic, too sick to be involved)

Question 17

What is a cohort?

Answer 17

Group of subjects

Question 18

What are the 2 types of cohort studies?

Answer 18

Prospective: measure exposure in the present, and outcome is assessed at some point in the future.
Retrospective: measures outcomes in the present, and exposure is assessed at some point in the past.

Question 19

Cohort Studies

Answer 19

Initial information on a group of people is collected at baseline or at the beginning. This cohort is then followed over time (normally many years) to quantify the health outcomes of each individual within it. Most cohort studies are prospective.

Question 20

What are some study examples that shaped the field?

Answer 20

Nurses health study
EPIC study
PREDIMED study
Finnish Twin Cohort study
Framingham study
Japan Collaborative Cohort Study

Question 21

Sample selection and follow-up (Cohort studies)

Answer 21

The sample is not always selected to represent the distribution within the whole population.
The sample may be weighted to maximise the heterogeneity of exposure or might be selected to minimise loss to follow-up.
The primary concern is to select a sample that will not be lost in the follow-up period.

Question 22

What are the strengths of cohort studies?

Answer 22

They provide the strongest evidence for a causal relationship because it’s unlikely that the measure of exposure is biased and measured before the outcome is known, and may therefore predict a causal pathway

Question 23

What are the limitations of cohort studies?

Answer 23

Loss to follow-up
Time consuming (years)
Expensive
In chronic diseases such as cancer, coronary heart disease, or diabetes: large numbers of people must be followed up for long periods before sufficient cases accrue to give statistically meaningful results.

Question 24

What are case reports?

Answer 24

Detailed description of one or more cases of interest.
Descriptive in nature.
The analysis is generally narrative in nature.
It can be used to generate ideas and new hypotheses, which can later be tasted.
We cannot say anything much about the relationship between diseases and diet/exposure from these types of writing.

Question 25

Explain Randomised Control Trials

Answer 25

2 o more groups of people are compared:

One (or more) experimental groups who receive a new standard treatment (which might be the best existing treatment, no treatment or a placebo). 

Information from the control group allows researchers to see whether the new treatment(s) are more or less effective than the current standard treatment. 

It is important that in a RCT, the two (or more) groups of people in a trial are as similar as possible, except for the treatment they receive. 

This is important because it means that researchers can be sure that any differences in outcomes between the groups are only due to the treatment received. 

Randomisation is the best way of ensuring that the results of trials are not biased by how participants in each group are selected.

Question 26

What is a confounding variable?

Answer 26

It is an extra variable that you didn’t or couldn’t account for within a study design.
The confounder falsely appears to be the cause of the outcome instead of the true cause.

Question 27

Example of a confounding variable

Answer 27

Is taking vitamins responsible for reduced CHD risk? Or do people who take vitamins have healthier lifestyles?

Question 28

What is a Systematic review?

Answer 28

A systematic review answers a defined research question by collecting and summarising all empirical evidence that fits pre-specified eligibility criteria

Question 29

What is a meta-analysis?

Answer 29

It is the use of statistical methods to summarise the results of these studies

Question 30

What are the characteristics of systematic reviews and meta-analyses?

Answer 30

Clearly stated objectives with pre-defined eligibility criteria for studies

Explicit, reproducible methodology 

A systematic search that attempts to identify all studies 

Assessment of the validity of the findings of the included studies (e.g. risk of bias) 

Systematic presentation, and synthesis of the characteristics and findings of the included studies

Question 31

What is the advantage of a case-control study?

Answer 31

They can be used to calculate the goods ratio between the odds of diseases in those exposed to the interest compared to odds of disease in those not exposed

Question 32

Which of the following statements about randomised controlled trials are correct?

Answer 32

They provide the strongest evidence that there is a causal link between exposure and outcome.
They can be used to decide if a diet programme or diet intervention is effective in achieving the stated outcomes.

Question 33

Which are the limitations of studies using an ecological study design?

Answer 33

They cannot prove the causes of disease.
They cannot show the temporal (over time) effect of exposure.
We cannot compare the risk of disease between the cases of the disease and those with the disease.