Quantitative - design, sampling and randomisation issues in randomised controlled trials

Question 1

What is the basic layout of a RCT?

Answer 1

study pps –> intervention group and control group –> outcome present in either group? (yes or no)

Question 2

what is the design of a RCT?

Answer 2

• all trials have an intervention
• PPs randomised to either recieve the intervention or not (control group)
• sometimes there’s an attention arm (similar to intervention but without ‘active’ ingredient)
• outcomes of interest = measured

Question 3

What are hypotheses?

Answer 3

• trials frquently start with them

- predictions about what they expect the intervention to achieve

Question 4

What is a null hypothesis (Ho)?

Answer 4

• there will be no difference between the control and intervention arms
• assumed true at the start and has to be disproved

Question 5

What are variables?

Answer 5

factors being investigated

Question 6

What is a dependant variable?

Answer 6

-often the outcome of interest e.g. healing time

Question 7

What is an independant variable?

Answer 7

-the intervention factor e.g. the dressing being used

Question 8

How else can the IV and DV be described?

Answer 8

like cause (effect of dressing - IV) and effect (healing time - DV)

Question 9

How is the population chosen in trial recruitment?

Answer 9

• target group researchers are interested in
• must be cleary defined (so know wwho results are applicable to)
• a sample will then be drawn (representative of the population)

Question 10

What critera do trials have when recruiting?

Answer 10

inclusion and exclusion

Question 11

What types of sampling are there?

Answer 11

probability and non-probability

Question 12

What is probability sampling?

Answer 12

• gives an unbiased sample where everyone who meets the criteria has a chance of selection
• choose the sample of those entering the trial and is different to them being randomly allocated to groups once in the trial

Question 13

What is non-probability sampling?

Answer 13

-non-random sampling and the chance of being selected cant be estimated

Question 14

What are the four types of probability sampling?

Answer 14

simple random, stratified random, cluster and systematic random

Question 15

What is simple random sampling?

Answer 15

• random selection of everyone in the population list

- rarely done because difficult to get population list

Question 16

What is stratified random sampling?

Answer 16

-put in groups according to characteristics (lile gender) then randomly selected

Question 17

What is cluster sampling?

Answer 17

-random selection of larger units (like hospitals) which PPs are then randomly selected from

Question 18

What is systematic sampling?

Answer 18

-random selection at predetermined intervals

Question 19

What are the factors affecting sample size?

Answer 19

• population factors - similarity of population to each other, expected rate of ‘event’, expected attrition (loss to study)
• design - number of variables, sampling
• measurement - sensitivity of measures
• practical factors - cost and convenience

Question 20

What is a power calcuation and when is it performed?

Answer 20

• used to calculate the minimum effect sample size that is likely to be detected in a study using a given sample size.
• also used to calculate the minimum sample size required so that one can be reasonably likely to detect an effect of a given size

Question 21

How are patients allocated to trial groups in RCTs?

Answer 21

-usually in a sequence over a period of time

Question 22

How are patients allocated in to arms of the trial?

Answer 22

-methods of randomising them into the arms of the trial (intervention or control) are similar to the random selection of the sample

Question 23

What are the three types of randomisation?

Answer 23

simple, block, stratified

Question 24

What is simple randomisation?

Answer 24

-‘tossing a coin’ but usually done with random number tables

Question 25

What is block randomisation?

Answer 25

-to keep numbers in each group close

Question 26

What is stratified randomisation?

Answer 26

-to balance chosen characteristics across the arms of the trial

Question 27

What does it mean when a trial is single blind?

Answer 27

- one person (usually PP) knows which arm of the trial thy are in (e.g. if its obvious what treatment they recieve) - perosn assessing the outcome doesnt know

Question 28

What does it mean when a trial is double blind?

Answer 28

neither PP or person assessing outcome knows the arm (e.g. placebo)