RT Article T1 Observer behavior as a potential source of reactivity: Describing and quantifying observer effects in a large-scale observational study of police JF Sociological methods and research VO 34 IS 4 SP 521 OP 553 A1 Spano, Richard LA English YR 2006 UL https://krimdok.uni-tuebingen.de/Record/1912639351 DO 10.1177/0049124106286331