To back up their conclusions, psychologists look for evidence in case studies and the results of experiments. They do not use expert opinion as evidence; direct quotations of what other psychologists have written are rare in psychology papers. Instead, papers focus on data (the results of experiments) and on the analysis of the results that the writer has collected.
Depending on their specialization, psychologists may ask questions that lead to quantitative or qualitative research and data. Quantitative research and data involve numerical measurement of phenomena; qualitative research and data involve interviews of subjects and the researcher’s verbal descriptions of observations.
Quantitative evidence might be facts and statistics: “Regional cerebral blood flow in a total of 26 areas predicted performance, and 20 of these areas predicted performance only in a single task”; “In a study on what motivates adolescents to quit smoking, 44.7% of the participants reported that they wanted to quit because their parents wanted them to.” Or it might be results of original experiments: “Fraudulent excuse scores were correlated with cheating scores (r = .37, n = 211, p < .0001).”
Qualitative evidence is usually examples and illustrations: “Many of the respondents believed girls’ tendency either to address indirectly or to avoid conflict was supported by adults, who expected them to be ladylike; when asked to define this term, they used such descriptors as ‘mature’ and ‘calm.’”