Engineers, particularly geotechnical engineers, often rely on judgement to integrate various controlling factors during design. Engineers learn heuristics as they gain experience, however these have limitations and their application is dependent on problem complexity. This paper sought to investigate the role of problem complexity on judgement by carrying out a survey of geotechnical practitioners. Two questions about design safety were posed: one regarding a tailings dam and another on a waste cover. Respondents were randomly divided into two groups, and for each of the two questions, one group received slightly more information than the other. For the tailings dam question, for which several factors control safety, responses, regardless of experience, were unaffected by providing more information on one primary variable. In contrast, for the cover design question, where fewer factors control safety, experienced respondents were strongly influenced by providing more information about one primary variable. This illustrates how judgement decisions, regardless of experience, are difficult for problems with several controlling variables. Worryingly, some experienced respondents provided with quantitative strength data, made unconservative estimates of cover stability. This highlights that even for simple problems, judgement-based decisions should be carefully interrogated.