Classification of conditions based on length of the prevalence period. Figure describes the impact of changing the length of the prevalence period on the classification of a patient condition into one of four mutually exclusive groups: condition present in both prevalence and incidence periods; condition present in prevalence period only; condition present in incidence period only; and condition present in neither prevalence nor incidence periods. (Left-hand box) P indicates that that condition is classified as prevalent and is excluded from the risk set for incidence. I indicates the condition is incident. The incidence is calculated as I/I + D, where D is the count of patients without the condition in either the prevalence or incidence period. Increasing the prevalence period (upper right-hand box) results in more conditions being classified as prevalent. Patients with the condition present in the incidence period only can be reclassified as having the condition present in both the prevalence and incidence periods (upper arrow within upper right-hand box). Patients without the condition in either the prevalence or the incidence period can be reclassified as having the condition in the prevalence period only (lower arrow within upper right-hand box). Patients move in the opposite direction when the prevalence period is shortened (lower right-hand box).