2.8 Comments on describing blinding
Blinding is when those involved in the study do not know information about the study.
Those involved in the study may not know:
- that they are in a study at all;
- the purpose of the study; and/or
- which comparison group they are in.
When participants are blinded in as many ways as possible, the internal validity of the study is increased. However, when people are the individuals, ethics requirements often mean that they need to know they are in a study and the purpose of the study.
Different individuals involved in the study can be blinded:
- A study can blind the researcher to knowing what comparison group the study individuals are in.
- A study can blind the participants to knowing what comparison group they are in.
- A study can blind the analysts to knowing what comparison group the individuals are in during analysis.
When as many participants are blinded as possible, the internal validity of the study is increased.
If only the participants are blinded, the study is called single blind. If both the researchers and participants are blinded, the study is called double blind. If the researchers, participants and the analyst are blinded, the study is called triple blind. However, for clarity, explicitly stating who or what is blinded is best. Blinding should be considered in all studies, when possible (and it is not always possible).
Blinding of participants does not just apply to people; it is also relevant with animals (Example 2.14 about Clever Hans).
Example 2.18 (Blinding) In a study comparing chest compressions with dominant and non-dominant hands of student paramedics (Cross et al. 2019), the article states that:
Participants were asked to participate in a study exploring general CPR performance but were blinded to the specific research question at any stage to reduce the chance of performance bias...
--- Cross et al. (2019), p. 2
Participants could not, however, be blinded to which group they were in (dominant hand on chest; non-dominant hand on chest). In this case, participants were only partially blinded.
Later, the article reports that:
This means that the analyst was blinded to the treatments.Data were analysed by a biostatistician blinded to group allocation.
--- Cross et al. (2019), p. 3
Example 2.19 (Double-blinding) In a cropping study comparing yields from modern and traditional cowpea crops in Tanzania, the researchers wanted to use a double-blind study.
To do so:
...it was important that the traditional and modern seed looked exactly the same---the seed types must be indistinguishable in terms of size and color.
While information about seed type may be gradually revealed as the crop matures in the field, this does not invalidate our design because key inputs were already provided.
Since the modern seed was treated with purple powder, we also dusted the traditional type, and clearly communicated this to the farmers---they knew that seed type could not be inferred from the color.
Bulte et al. (2014), p. 817--818; line breaks added