What method of data collection did Laud Humphrys use in his study of homosexual encounters among men in public restrooms, published as the book Tearoom Trade?

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Laud Humphrys used covert participant observation in his study of homosexual encounters among men in public restrooms. He disguised himself as a typical user of public restrooms and observed the behavior of men engaging in homosexual encounters. He also interviewed some of the men involved in these encounters without revealing his identity.

What Method of Data Collection Did Laud Humphrys Use in His