When the researcher does not know the identity of the experimental and placebo groups, the study is termed as:

When the researcher does not know the identity of the experimental and placebo groups, the study is termed as: Correct Answer Double-blind

The experimental research is a scientific approach to research in which the researcher manipulates/controls one or more variables and measures its effect on the other variables. The strength of this research is its internal validity (causality). However, the influence of experimenter bias can threaten the internal validity of the research.

Controlling the Researcher’s Bias: The element of experimenter-bias effect can be controlled through the process of blinding. It is the process used in experimental research by which study participants, persons providing the treatment, data collectors and data analysts are kept unaware of group assignment (control vs treatment). There can be varying degrees of blinding such as single-blind, double-blind, triple-blind, etc.

1. Single-blind: 
   1. ​In this case, the participants are unaware if they are part of the experimental group or the control group. It can be used, for example, during the test phase of a new drug, where the subjects do not know if they are actually receiving it or not.
2. Double-blind:
   1. It describes an experimental procedure in which neither the participant nor the experimenter is aware of which group (experimental or control) each participant belongs to.
   2. It uses a rigorous way of experimenting in an attempt to minimize subjective biases on the part of the experimenter and on the part of the participant and obtain a more valid result.
   3. It is most commonly utilized in medical studies that investigate the effectiveness of drugs.

NOTE:

Longitudinal Research: This type of research is used to study the same sample over a longer period of time. These may be used to study behavioural changes, attitudinal changes or religious effects that may have a long time effect on the selected sample.

For example, to study the impact of the Reservation Policy of the Govt. of India to overcome inequalities since independence. Types of longitudinal surveys include:

Hence, when the researcher does not know the identity of the experimental and placebo groups, the study is termed as a double-blind study.