What can epidemiological studies tell us?
In general, incidence is more informative when attempting to determine the cause of a disease, or whether the risk of a disease is changing with time.5 The study of MS is, however, a special case, and there are problems associated with obtaining serial incidence data which are comparable, therefore it is easier to rely on prevalence data.5 The nature of MS makes it difficult to estimate incidence with certainty. MS is an uncommon disease so, unless the population being studied is very large, or the duration of assessment is very long, only a small number of new cases will occur. If estimates of incidence are based on small numbers they are too easily influenced by random variation to be of value.5 The variability of MS symptoms, difficulty in recognizing the initial symptoms, and the length of time between their resolution and the first relapse all make accurate diagnosis difficult. Hence, there can be a long delay between the onset of MS and confirmation of the diagnosis.5 Since incid
