This journal article discusses police discretion from a psychological or behavioural perspective. It also outlines research that constructs scales for measuring police attitudes toward discretion. 

Richard Wortley, 'Measuring Police Attitudes toward Discretion' (2003) 30(5) Criminal Justice and Behavior 538

The law places limits on how police operate. Within those limits, police have a large amount of discretion about how they act. The way police use their discretion shapes people’s experiences with the criminal legal system and has been repeatedly linked to the over-representation of Aboriginal people in custody.

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Richard Wortley, 'Measuring Police Attitudes toward Discretion' (2003) 30(5) Criminal Justice and Behavior 538

Police discretion