An assistant or trainee psychologist is working towards full qualification and always practises under the supervision of a qualified psychologist, though the two roles differ in what stage of that journey they represent. An assistant psychologist holds a psychology degree and supports assessment, data collection, structured interventions and general service work, often as a step towards applying for doctoral training.
A trainee psychologist, by contrast, is already on a doctoral training programme and sees clients directly as part of that training, working through real cases under close supervision as the core method by which the doctorate is completed. Neither role is independently registered with the Health and Care Professions Council, and neither should be working with clients unsupervised.
Supervised is not lesser
It is worth being clear that supervision does not mean a lower standard of care. A trainee's work is discussed in detail with an experienced supervisor, often session by session, which in some respects means more oversight than a fully qualified practitioner working alone receives. Understanding the arrangement, though, helps make sense of who is actually accountable for a piece of care and what the qualification pathway looks like from here.