Explain whether below experiment is single blind, double blind or neither.
To test an herbal treatment for depression, 100 volunteers who suffered from mild depression were randomly divided into two groups. Each person was given a month's supply of tea bags. For one group, the tea contained the herb mixed with a spice tea; for the other group, the bags contained only the spice tea. Participants were not told which type of tea they had and were asked to drink one cup of the tea per day for a month. At the end of the month, a psychologist evaluated them to determine whether their mood had improved. The psychologist did not know who had the tea with the herbal ingredient added.

In a double blind experiment, neither the researchers nor the participants are aware about which group is receiving the experimental treatment. This is done in order to prevent biases in the participants' judgment (the placebo effect), and prevent the researchers from giving accidental cues.

The greatest advantage of double blind studies is they have greater reliability and validity than other types of studies.