The manner in which heuristics and biases influence clinical decision-making has not been fully investigated and the methods previously used have been rudimentary.

Two studies were conducted to design and test a trial-based methodology to assess the influence of heuristics and biases; specifically, with a focus on how practitioners make decisions about suitability for therapy, treatment fidelity and treatment continuation in psychological services.

Study 1 (N=12) used a qualitative design to develop two clinical vignette-based tasks that had the aim of triggering heuristics and biases during clinical decision making. Study 2 (N=133) then used a randomized crossover experimental design and involved psychological wellbeing practitioners (PWPs) working in the Improving Access to Psychological Therapies (IAPT) programme in England. Vignettes evoked heuristics (anchoring and halo effects) and biased responses away from normative decisions. Participants completed validated measures of decision-making style. The two decision-making tasks from the vignettes yielded a clinical decision score (CDS; higher scores being more consistent with normative/unbiased decisions).

Experimental manipulations used to evoke heuristics did not significantly bias CDS. Decision-making style was not consistently associated with CDS. Clinical decisions were generally normative, although with some variability.

Clinical decision-making can be ‘noisy’ (i.e. variable across practitioners and occasions), but there was little evidence that this variability was systematically influenced by anchoring and halo effects in a stepped-care context.