Lecture 1 - Study design

Question 1

What is unique to a descriptive study

Answer 1

no comparison groups

Question 2

What does an observational study not have?

Answer 2

it has a comparison group but no intervention

Question 3

What are the features of a clinical trial

Answer 3

• randomisation
• concealed allocation
• placebo
• blinding
• intention to treat

Question 4

What is randomisation?

Answer 4

allocation of treatments to patients using some random process

Question 5

What is the purpose of randomisation?

Answer 5

it will produce study groups which are comparable with respect to known and unknown factors which are likely to influence outcomes

Question 6

What are the benefits of randomisation?

Answer 6

• observed difference in the two groups will be due to effects of treatment
• fair comparison between groups
• statistical analysis only valid if this takes place

Question 7

What is a confounding factor

Answer 7

factors that influence treatment and outcome measures

-include demographic characteristics / prognostic factors

Question 8

What is restricted randomisation and give an example

Answer 8

number of patients per treatment chosen in advance

- block randomisation

Question 9

benefits of stratification?

Answer 9

useful for small sample sizes as simple random allocation would not mean risk factors would be distributed equally

Question 10

what is the aim of a clinical trial

Answer 10

minimise bias and confounding

Question 11

what is selection bias and is it removed

Answer 11

enrolling a certain patient is influenced by which treatment they might receive
-removed using sealed envelopes

Question 12

What can knowledge of placebo effect do?

Answer 12

influence subject QoL/ pain levels

Question 13

What is intention to treat?

Answer 13

analysing data if they received treatment or if they did not

Question 14

Why is intention to treat useful?

Answer 14

accurately reflects ‘real life’

Question 15

What is per protocol analysis

Answer 15

reported as secondary outcomes measures the effect on who actually received the treatment

Question 16

What is the pinnacle of evidence-based medicine

Answer 16

1. systematic reviews and meta-analyses

2. randomised controlled double blind studies

Question 17

How can results from a trial of 2 treatment with 2 categories be summarised

Answer 17

2 x 2 frequency table

Question 18

how is risk of failure calculated?

Answer 18

failures of A / failures of A + success of A

- failure / total of A

Question 19

How is odds of failure calculated?

Answer 19

failure of A / success of A

Question 20

define risk reduction

Answer 20

difference in risk between the two treatment groups

Question 21

define number needed to treat

Answer 21

reciprocal of the risk reduction

- number of patients needed to be treated to prevent a single end point

Question 22

what is the odds ratio

Answer 22

a/b divided by c/d
useful to describe relative difference between treatment groups
-logistic regression produces odds ration

Question 23

What can’t be guaranteed by observational studies?

Answer 23

-other important factors causally related ti both disease and exposure

Question 24

Can causation be inferred from an observational study

Answer 24

no due to bias and confounding variables

Question 25

what is a cohort study?

Answer 25

• prospective in nature
• population subgroup is defined and followed up over time
• exposure status is measured at the beginning of the study
• disease status is observed and recorded during follow-up

Question 26

what is a case-control study?

Answer 26

-retrospective
-subjects w disease (cases)
subjects wo disease (controls)

Question 27

How do case-control studies draw their conclusions?

Answer 27

• frequency of exposure ascertained in each group
• higher prevalence of exposure among controls suggest exposure might be a risk factor
• selection /recall bias present

Question 28

Cohort vs case-control

Answer 28

• cohort is more expensive and results can take a long time to materialise
• cohort is unsuitable for rare diseases
• cohort can confirm findings in case-control

Question 29

what is the risk ratio

Answer 29

risk of the disease in the exposed group relative to the unexposed group

Question 30

why must odds ratio be used in case-control studies

Answer 30

odds of disease cannot be calculated directly

Question 31

what can a small sample size mean?

Answer 31

• lack of statistical power

- if there is a real difference you won’t detect it