2.1.7 Blinding
THIS ITEM IS UNDER CONSTRUCTION
A. Background & Definitions
Blinding refers to the masking of the treatment allocation for person(s) who perform the experiment, collect data and assess outcome. Blinding aims to make sure that someone has no knowledge about treatment allocation that may systematically influence his/her performance. The intended result is the equal treatment (as far as possible) of all experimental units (animals, subjects or samples) in the experiment.
In the discussion below, experimental groups refer to all groups involved in an experiment, for example: control, sham, treated with drug A, treated with drug B, etc.
Group allocation describes which experimental unit (animal, subject or sample) has been allocated to which experimental group.
The group allocation, actions and outcome assessments are ‘blinded’. People are ‘blind’ to particular information.
B. Guidance & Expectations
Effective blinding requires at least 2 people, one blinded person (unaware of experimental condition) and an unblinded person (knows the experimental condition and the blinding code). The unblinded person is the keeper of the blinding code which needs to be concealed until all processes under blinding are concluded.
Describe who was aware of the group allocation at the different stages of the experiment (during the allocation, the conduct of the experiment, the outcome assessment, and the data analysis). Any process using humans as perceptors, raters or interpreters needs to be blinded as long as decision-making is concluded.
Allocation concealment in animal experiments
The investigator shall not be aware and/or have the choice to which treatment group an animal is allocated to. This prevents selection bias. Therefore, the assignment to a specific group needs to be concealed and every animal should have the same chance to be assign to each of the groups. This can be achieved separating the assignment of animal_IDs to each animal (e.g. individual ear mark or subcutaneous chip) and randomization of treatments (see randomization) into two independent processes and then merge the two.
Effective blinding code
There are certain strategies to apply blinding. The most simple one is the blinding by numbers or alphabet letters. This can be problematic when human processing and rating is involved in outcome assessment. The assessor may not know the condition behind the code but the knowledge of a group affiliation of a sample is a bias that can influence rating. This should be avoided.
Sometimes blinding conditions are added to another code such as animal_ID, measurement_ID or file name. Watch out for hidden cues in such IDs, containing temporal or sequential information that could increase rater bias. Also metadata, such as creation date and time of a file containg measurements can give away experimental conditions.
If possible, check whether it is possible to use a blinding scheme without repeating codes. This is can be easily done with alphanumeric code consisting of 4 letter/number combinations, such as T7Z4. Such codes can be easily generated in Excel
=CHAR(RANDBETWEEN(65;))&RANDBETWEEN(0;9)&CHAR(RANDBETWEEN(65;))&RANDBETWEEN(0;9)
Enter this formula in a row of cells for which you need coded samples. Copy the outcome to another worksheet with the command Past Special-->Paste Values.
Planning to blind a study
To effectively blind a study, create a sequence containg all experimental steps of the study. In the neighboring headers indicate the name of each person involved in the conduct and analysis of the study. For each experimental step document for each person whether they are blinded or not blinded to the condition or not involved at all. Drawing this out systematically creates a transparent workflow of blinded and unblinded personnel and shows when unintended unblinding might occur. Add the final table to all experimental documentation and reporting.
RISK ASSESSMENT
- Under some circumstances, unintentional unblinding (e.g. due to a different appearance of a positive control in solution or suspension) may be a risk to be assessed and/or controlled
- Experimental treatments may produce adverse effects and attending veterinarians and animal care stuff may need to be informed in advance about the possibility of such adverse effects occurring and, if necessary, have emergency access to the blinding protocol.
PLEASE DO NOT FORGET
- Blinding is sometimes not possible especially when certain cues cannot be blinded, such as skin color of transgenic mice or color of a solution in a well. It is important to document this and to communicate in reports where blinding could or could not be achieved.
- Unblinding of the experimental conditions should be done when all blinded processes for the entire study are concluded. Early and partial unblinding for "checking" should be avoided and if necessary, be part of the protocol.
- Control group(s) (e.g., positive control group) should not be excluded from the blinding procedure.
- Provide training on how to apply the blinding procedure.
C. Resources
Guidelines on reporting of blinding (in vivo research): ARRIVE 2.0
Reading material:
Handbook of Experimental pharmacology chapter on randomization and blinding [1]
back to Toolbox
Next item: 2.1.8 Randomisation
