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Taks 2: randomization
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Task 2 – randomizationLearning goals:
1) What is randomization and concealed allocation? (relation)
Randomization
Randomization is a process by which each participant has the same chance of being assigned to
either intervention or control. Neither the trial participant nor investigator should know what the
assignment will be before the participant’s decision to enter the study. Otherwise, the benefits of
randomization can be lost.
Randomization tends to produce study groups comparable with respects to known and unknown risk
factors, removes investigator bias in the allocation of participants and guarantees that statistical tests
will have valid false positive error rates.
Two form of bias are of concern:
- Selection bias = occurs if the allocation process is predictable
- Accidental bias = can arise if the randomization procedure does not achieve balance on risk
factors or prognostic variates. For larger studies, the chance of accidental bias is negligible
(law of numbers)
The report of the trial should contain a brief, but clear description of the randomization used.
To accomplish a valid randomization, it is recommended that an independent central unit is
responsible for developing the randomization process and making the assignments of participants to
the appropriate group.
Concealed allocation
Allocation concealment is the technique of ensuring that implementation of the random allocation
sequence occurs without knowledge of which patient will receive which treatment.
2) What are the different types of randomization? (pros + cons / when to use)
FIXED RANDOMIZATION PROCEDURES
Fixed allocation procedures assign the interventions to participants with a prespecified probability
(usually equal) and the allocation probability is not altered as the study progress.
Types of fixed allocation randomization:
- Simple randomization
One simple method is to toss an unbiased coin each time a participant is eligible to be randomized.
Example: if the coin turns up heads. The participant is assigned to group A
Using this procedure, approximately one half of the participants will be in group A and one half in
group B. In practice, for small studies, instead of tossing a coin to generate a randomization schedule,
a random digit table on which the equally likely digits 0-9 are arranged by row and columns is usually
used to accomplice simple randomization.
For large studies, a more convenient method for producing a randomization schedule is to use a
random number producing algorithm, available on most computer systems. A simple randomization
, Task 2 – randomization
procedure might assign participants to group A with probability p and participants to group B with
probability 1 – p.
Advantages:
- Easy to implement
Disadvantage:
- There could be a substantial imbalance between the groups, especially if the sample size is
small. These imbalances reduce the ability to detect true difference between the two group
and may lead to some loss of credibility for the trial.
- Selection bias not totally blinded
For this reason, simple randomization is not often used, even for large trials.
- Block randomization
Block randomization avoids serious imbalance in the number of participants assigned to each group
(which could occur in the simple randomization procedure). Block randomization guarantees that at
no time during randomization imbalance will be large and that at certain points the number of
participants in each group will be equal.
Of participants are randomly assigned with equal probability to groups A or B, then for each block of
even size (4, 6, 8); one half of the participants will be assigned to group A and the other half to group
B. The order in which the interventions are assigned in each block is randomized and this process is
repeated for the consecutive blocks of participants until all participants are randomized.
Example: in case of block size 4 with two group; there are six possible combinations of group
assignments AABB, ABAB, BAAB, BABA, BBAA, ABBA
One of these arrangements is selected at random and the four participants are assigned accordingly.
This process is repeated as many times as needed.
Another method of blocked randomization may also be used. In this method for randomizing the
order of assignments within a block of size X, a random number between 0 and 1 for each of the X
assignments (half of which are A and the other half B) is obtained.
The assignments than are ranked according to the size of the random numbers. This leads to the
assignment order ABAB. This process is repeated for another set of four participants until all have
been randomized.
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