The shift to Vocational Education and Training (VET) for the training of the Australian residential aged care workforce has resulted in significant variance in the competencies at graduation of those caring for people with dementia. Competence may be also be enhanced, or conversely, decline over time as a result of implicit learning gained from workplace experience. Evaluating the skills and knowledge of the workforce is a challenge for supervisors. A single valid assessment tool that can evaluate staff knowledge across the competence continuum, from personal care workers to registered nurses, is required. An existing case study-based summative test of knowledge was chosen to be used in conjunction with a newly-developed marking rubric. The rubric was designed to differentiate between answers from different levels of staff. The assessment task and rubric were tested on a representative sample of the competent Australian residential aged care workforce with the appropriate skills mix. Evidence of the adequacy with which the test represents the national dementia-care competency, the responses of test users to the problem-solving content of the case study and the validity of the scores have been established. The validated scores were found to establish normal distributions, or ranks of knowledge for the different levels of staff. Potentially these permit the evaluation of the performance of groups of staff in residential aged care facilities against the distributions of scores. (Contains 1 figure and 4 tables.)