Complex sentence: One of the tests was organized by the parapsychologist Joseph Rhine at Duke University in 1933 which involved Zener cards with certain symbols placed in a sealed envelope and participants were asked to guess their contents.
Split into simpler sentences: One of the tests was organized by Joseph Rhine at Duke University in 1933 which involved Zener cards. Certain symbols that were placed on the cards and sealed in an envelope, and participants were asked to guess their contents.