After reading the cochrane handbook and some articles I am still confused between those two domains.
Allocation concealment - A technique used to prevent selection bias by concealing the allocation sequence from those assigning participants to intervention groups, until the moment of assignment. Allocation concealment prevents researchers from (unconsciously or otherwise) influencing which participants are assigned to a given intervention group.
Blinding (masking) - The practice of keeping the trial participants, care providers, those collecting data, and sometimes even those analyzing data unaware of which intervention is being administered to which participant. Blinding is intended to prevent bias on the part of study personnel. The most common application is "double-blinding", in which participants, caregivers and those assessing outcome are blinded to intervention assignment. The term "masking" may be used instead of blinding.
In short, in the case of Allocation concealment, both patient and researchers will know which treatment is being given. Of course, the evaluation of results may cause bias of the researchers, or even some knowledgeable patients. In the case of Single blind trial, the patients will not know whether they are in the control arm or trial arm; (some patients in control arm may be unhappy that they are not receiving the new drug!). Double blind means both patients and researchers are blind to the treatment.
Narayanan Bhattathiri
Allocation concealment - A technique used to prevent selection bias by concealing the allocation sequence from those assigning participants to intervention groups, until the moment of assignment. Allocation concealment prevents researchers from (unconsciously or otherwise) influencing which participants are assigned to a given intervention group.
Blinding (masking) - The practice of keeping the trial participants, care providers, those collecting data, and sometimes even those analyzing data unaware of which intervention is being administered to which participant. Blinding is intended to prevent bias on the part of study personnel. The most common application is "double-blinding", in which participants, caregivers and those assessing outcome are blinded to intervention assignment. The term "masking" may be used instead of blinding.
In short, in the case of Allocation concealment, both patient and researchers will know which treatment is being given. Of course, the evaluation of results may cause bias of the researchers, or even some knowledgeable patients. In the case of Single blind trial, the patients will not know whether they are in the control arm or trial arm; (some patients in control arm may be unhappy that they are not receiving the new drug!). Double blind means both patients and researchers are blind to the treatment.
Narayanan Bhattathiri
Narayanan gave a wonderful answer. I just want to clarify one point. Double-blinding is not the most common approach, simply because it is not always (mostly not) possible. For example, a study in which patients must perform exercises, cannot be double-blinded, as both the researcher and subject know that the subject is exercising. Double-blinding is suitable for interventions where a placebo can be given and thus neither researcher nor subject would know if it were the placebo or actual drug.
I agree with the above responses, with one slightly different take- blinding doesn't just refer to the treatment- it can refer to the group into which a participant falls. So you can have mutiple types of blinding:
Blind allocation (of which concealed allocation is a type) where the person allocating the participant to a group is blinded to which group (treament or control) they are allocating them to.
Patient blinding, where the patient is unaware of whether they are in the treatment or control group.
Therapist blinding where those who are delivering the treatment are blinded to the participant's group
Assessor blinding, where those measuring the key outcome measure are unaware of group status.
You can even have analysis blinding where those analysing the data are unaware of group allocation.
As Ariel pointed out there are situations where blinding more than one is just not feasible. But if you hear of triple blinding with concealed allocation, you know that many steps have been taken to ensure anonymity of the participants group status.
This was very helpful. Now, that I've read these comments, what I was originally told makes more sense and is a simplifying principle:
1. Allocation Concealment happens before randomization (And can be part of the randomization process).
2. Blinding happens after Randomization
It all boils down to what kind of bias you are trying to control for
Allocation Concealment is a method to prevent selection bias
Blinding is a method to prevent Several other kinds of bias:
- Observer bias (biases from the tester)
- Placebo effect/Nocebo effect (biases from the subject)
- biases of data analysis and interpretation
After reading this thread I found a great article that highlights how bias is prevented in various arms of a research project. (hyperlink added)
Thanks everyone for helping me out - Clinical epidemiology starts tomorrow!
-Mike B
M1 Med Student
https://wiki.ecdc.europa.eu/fem/w/fem/preventing-bias.aspx
Thanks Mike Birkhead, you have summarised nicely. The key difference is that allocation concealment is done before/along with randomisation, while blinding is usually happened after randomisation.
cheers!
Rakib
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