Critical Numbers

Question 1

Sampling

Answer 1

We take a representative sample from the population of interst

We describe our sample using descriptive statistics

We make inference about our population using inferential statistics

Samples may be randomly or non-randomly selected

There are different methods of random and non-random sampling

Important thing is sample must represent population of interest
This helps make our results generalisable

When certain subgroups from the target population are over/under-represented the sample may be biased
e.g. GP survey which excludes centres in lower socioeconomic status areas

Question 2

Bias

Answer 2

Bias’ arises when imperfections in the research process cause our findings to deviate from the truth

Bias can occur in all studies

It can occur intentionally or unintentionally

It impacts upon the validity and reliability of study findings
Put simply, it can distort results

It is our responsibility to minimise bias in our research
and consider it when critically evaluating the research of others

Question 3

Types of bias

Answer 3

Sampling bias

Recall bias

Information bias

The ‘Hawthorne’ effect

Attrition bias

Question 4

Sampling bias

Answer 4

sample does not represent population of interest

Question 5

Recall bias

Answer 5

inaccurate recall of past events/exposures/behaviours

Question 6

Information bias

Answer 6

incorrect measurement e.g. miscalibrated machine

Question 7

The ‘Hawthorne’ effect

Answer 7

participants change their behaviour when they know they are being observed

Question 8

Attrition bias

Answer 8

differential dropout from studies e.g. sicker participants drop out so our outcome is only measured on healthier participants

Question 9

Cofounding

Answer 9

Confounding, if unaccounted for, is a form of bias

Confounding variables obscure the real effect of an exposure on an outcome

They may suggest that there is a relationship between two factors when there isn’t actually, or vice versa

Question 10

What is a cofounder?

Answer 10

A cofounder is related to both exposure and outcome

But is not on the casual pathway

E.g a high salt diet can cause high blood pressure which can lead to stroke. So blood pressure is NOT a cofounding factor in the relationship between deit and stroke, rather than a mechanism

Question 11

Correlation does not imply causation

Answer 11

Potential cofounders must be considered in the design, analysis and interpretation of studies

Question 12

Studies can be classified according to whether they are:

Answer 12

Experimental – the researchers have intervened in some way

Observational – the researchers have not intervened, merely observed

Question 13

Observational studies can be:

Answer 13

Retrospective – looking back into the past

Cross-sectional – a single snapshot in time

Prospective – following up over time

Question 14

Randomised controlled trials

Answer 14

Randomly allocate participants to different interventions and follow up

Experimental

Prospective

Slide 23

Question 15

There are variations on the standard RCT design

Answer 15

Cluster randomised trails

Crossover trials

Multi-arm and factorial trials

Adaptive design trials

Regardless of design, all clinical trials are monitored throughout to ensure participant safety and integrity of results.

Question 16

Cluster randomised trials

Answer 16

participants randomised in groups (e.g. by GP centre or therapist) rather than at the individual level

Question 17

Crossover trails

Answer 17

participants receive both interventions in a randomised order

Putting people on drug A and swithcing them onto drug B half way through but if drug A has already cured means trial can not continue so we use this on people with chronic illnesses

Question 18

Multi arm and factorial trials

Answer 18

accruing information is used to inform planned design adaptations

Question 19

Pros of RCT

Answer 19

Gold standard’

Randomisation reduces potential for confounding

Can reduce bias via control and blinding

Can determine causal effects

Question 20

Cons of RCT

Answer 20

Randomisation can be unfeasible or unethical (can you think of any examples?)

Require expert management and oversight, particularly for ‘high risk’ interventions

Expensive

Question 21

Co-hort studies

Answer 21

Non-randomised

Observational

Typically prospective

Question 22

Pros of cohort studies

Answer 22

Useful when random allocation not possible

Can work for rare exposures – select participants on the basis of exposure

Can examine multiple outcomes

Question 23

Cons of cohort studies

Answer 23

May require long follow-up

Can be expensive

Not ideal for rare outcomes

Question 24

Case- control studies

Answer 24

Non-randomised

Observational

Retrospective

Question 25

Pros of case

Answer 25

Faster - use past data so do not require long follow-up Useful for rare outcomes – select participants on the basis of outcome Cheaper

Question 26

Cons of case-control studies

Answer 26

More prone to bias or poor quality data Harder to show causal relationship Not ideal for rare exposures

Question 27

Cross sectional studies

Answer 27

Non-randomised Observational Single time point

Question 28

Pros of cross-secctional studies

Answer 28

Relatively quick Cheap Can assess multiple exposures/outcomes

Question 29

Cons of cross sectional studies

Answer 29

Susceptible to bias Cannot prove causality Not ideal for rare exposures/outcomes

Question 30

Ecological studies

Answer 30

The unit of observation is group (aggregate) rather than individual e.g. Electoral ward, country E.g. Higher intake of fatty foods and higher rates on breast cancer in one country relative to another. This does not mean that those poeple who eat more fatty foods are the ones with cancer

Question 31

Pros of ecological studies

Answer 31

Large-scale comparisons Can quantify geographical or temporal trends

Question 32

Cons of ecological studies

Answer 32

Ecological fallacy Cannot make inference at the individual level

Question 33

The heirachy of evidence

Answer 33

Slide 34

Question 34

PICO

Answer 34

Population Intervention Comparison Outcome used to formulate a research question