Population-Level Study Design

Question 1

What does the practice of evidence-based medicine integrate?

Answer 1

1. Individual clinical expertise 2. The best available external clinical evidence 3. Patient values and expectations

Question 2

What are the steps taken during evidence-based medicine?

Answer 2

1. ASK: Formulate a research question 2. ACCESS: Find and retrieve the ‘best evidence’ 3. APPRAISE: Consider the evidence for its validity and relevance 4. APPLY: Integrate the results into clinical practice 5. ASSESS: Evaluate the effectiveness

Question 3

Quality of evidence pyramid

Answer 3

Question 4

What is the highest quality of evidence that can be used?

Answer 4

Systematic reviews

Question 5

What are the 4 types of evidence? How do they differ?

Answer 5

1. Description
2. Prediction
3. Causal interference
4. Qualitative

each require different methods & philosophies – none is ‘better’, they’re just different!

Question 6

What is causal inference?

Answer 6

the process of drawing a conclusion about a causal connection based on the conditions of the occurrence of an effect:

*

Question 7

What is the difference between inference of association and causal inference?

Answer 7

Causal inference analyses the response of the effect variable when the cause is changed

Question 8

What is descriptive evidence?

Answer 8

Used to describe characteristics of a population or phenomenon being studied. It does not answer questions about how/when/why the characteristics occurred … cannot describe what caused a situation.

• What happened?
• Who was affected?
• People with X had Y

Question 9

What is prediction evidence?

Answer 9

a prediction is made about the outcome of a future event based upon a pattern of evidence

• What will happen?
• Who will be affected?
• People with X are more likely to have Y?

Question 10

What is qualitative evidence?

Answer 10

Qualitative evidence provides richer, deeper and broader information based on a few individuals or case examples. This type of evidence is valuable for describing how and why.

• What matters…?
• Why does it matter?
• How can we effectively change X…?
• Should we change X?

Question 11

Causation of infectious disease example

Answer 11

Question 12

Deterministic vs probabilistic?

Answer 12

A deterministic model does not include elements of randomness. Every time you run the model with the same initial conditions you will get the same results.

A probabilistic model includes elements of randomness. Every time you run the model, you are likely to get different results, even with the same initial conditions (e.g. smoking not always leading to early death)

Question 13

Example of probabilistic

Answer 13

Question 14

Why is studying individual people useless with probabilistic events? Why are groups studied instead?

Answer 14

People are very different. Even the same people can respond very differently at different times.

We therefore must work with groups, and use probability and statistics to describe, predict, and make causal inferences.

Question 15

As the population is impossible or impratical to study, what is done instead?

Answer 15

so we usually study samples and try to generalise

Question 16

What is clinical researd focused on?

Answer 16

Focused on understanding and helping individuals

Question 17

What is population health research focused on?

Answer 17

Focused on understanding and helping populations

Question 18

Examples of populations

Answer 18

1) . The (total) inhabitants of a given geographical area: world, continent, country, county etc
2) . The universe from which a sample is drawn: typically a group of people (UK adults, Scottish children with type 1 diabetes etc) OR can be any collection of units (hospitals in England, births in Europe, total GP consultations in a medical practice over October 2017 etc)

Question 19

What is a census?

Answer 19

If your sample is the total population

Question 20

What is a population-based study?

Answer 20

If your study population = a total population of a country or area

Question 21

Types of studies

Answer 21

Question 22

Case report vs case series?

Answer 22

A case report is a detailed report of the diagnosis, treatment, response to treatment, and follow-up after treatment of an individual patient.

A case series is group of case reports involving patients who were given similar treatment.

Question 23

Why are case series beneficial?

Answer 23

Allows for description of natural history & prediction of prognosis:

–Frequency and range of symptoms

–Typical duration

–Typical survival

–Range of prognoses

–Predictors of prognosis

Question 24

What are register-based studies?

Answer 24

A special type of case-series

• Disease registers count and collect information on people diagnosed with a particular disease
• These are commonly population-based, so you can estimate the disease occurrence per population

Question 25

What is a cross-sectional study?

Answer 25

A study of a group of people at a single point in time (i.e. studying a cross section of the population in time) May involve: * Directly surveying or measuring a group of people * Gathering routinely-collected data

Question 26

Example of a cross sectional study

Answer 26

–Outcome: % of moustachioed medical leaders –Population: Clinical department leaders (n=1018) at the top 50 US medical schools funded by the National Institutes of Health. Proportion of female department leaders was 13.5% (137/1018) The proportion of moustachioed department leaders was 18.7% (190/1018)

Question 27

Descriptive studies usually also report their results broken down (‘stratified’) into subgroups. What are common characteristics for stratification? What is the purpose of this?

Answer 27

* Age * Sex * Socio-economic position * Self-reported ethnic group * Marital status * Occupation This provides more relevant estimates for these groups and individuals

Question 28

What are cohort studies?

Answer 28

Cohort studies examine groups of people over time

Question 29

Example of cohort study

Answer 29

–Population: all adult inpatients with lab-confirmed COVID-19 from two hospitals in Wuhan, China –Outcome: in-hospital death * Study: 191 patients were observed until discharge or death (by Jan 31 2020) * Events: 137 patients were discharged; 54 died in hospital during study period * Findings: Increased odds of in-hospital death for older age, higher Sequential Organ Failure score, and d-dimer \> 1μg/mL

Question 30

How are hypotheses generated?

Answer 30

Descriptive studies often present multiple stratified results, to generate hypotheses * Common variables include: * Individual-level characteristics –Age, sex, socio-economic position, ethnic group, marital status, occupation •Area-level characteristics –Countries, regions, latitude, urban vs rural areas, more vs less deprived areas, •Temporal characteristics –Secular trends (i.e. over time) –Dynamic or seasonal trends (e.g. month, season, day of week)

Question 31

What are case-control studies?

Answer 31

Examine groups of people over time retrospectively Example: **'UK case control study of smoking and risk of amyotrophic lateral sclerosis'** – Population: people diagnosed with ALS between 2008 and 2013 (vs. matched controls) – Outcome: risk of ALS * Survey: smoking status collected using environmental questionnaires * Participants: 388 records with full smoking history * Findings: Weak association between current smoking and risk of ALS; no increased risk with comprehensive smoking index (CSI) -\> no evidence of dose-dependence with higher levels of lifetime smoking

Question 32

What are the advantages of case-control studies?

Answer 32

–Good to study rare diseases –Can investigate multiple exposures at a time –Cheaper and less time consuming than a long-term cohort study

Question 33

What are the disadvantages of case-control studies?

Answer 33

–Recall bias –Need for suitable control group –Can establish correlation but NOT causation

Question 34

What are ecological studies?

Answer 34

A study that examines variations between geographical areas * The 'units of analysis' are not individual people but **areas** * Summary features of these areas are then compared to generate hypotheses

Question 35

Are ecological studies good for testing hypotheses?

Answer 35

NO - but can be useful for **generating** hypothesis

Question 36

Example of ecological study: The association between ultraviolet B irradiance, vitamin D status and incidence rates of type 1 diabetes in 51 regions worldwide

Answer 36

–Population: 51 regions across the world –Outcome: Type 1 diabetes –Exposure: Latitude \> Ultraviolet irradiance & vitamin D status