(taxis and tow trucks are common). in some cases, prior dispatch experience might involve one of the other emergency services. in some cases, although not a great many, trained paramedics may become emds. this may happen as the result of a work-related injury resulting in disability, or it may simply be that a paramedic completes training and then finds that field work is not to their taste, or that they lack the ability to perform the physical aspects of being a paramedic (e.g. heavy lifting) without fear of injury. there is some debate, particularly between paramedics and emds, as to whether prior training as a paramedic actually constitutes an advantage or an unnecessary distraction from the emd job function. valid perspectives exist on both sides of the debate. in large ems systems, emds may follow a progressive career path. actual dispatching may lead to training positions, and from there to supervisory or even managerial positions within the dispatch centre. emds may move from smaller systems to larger systems, just as paramedics sometimes do, in search of advancement opportunities, or economic improvements. in some cases, emds may choose to retrain and move to fire dispatch (efd), to police dispatch (epd), or to the 9-1-1 call centre. the inequities in the sheer amount of training required for certification make advancement of emds to paramedic status an extremely infrequent event. the required additional training, depending on the jurisdiction, might take two years or more, part-time, in addition to regular full-time employment, putting this option beyond the reach of all but the most determined. typically, emds who are also certified paramedics tend to have been paramedics first.