When the underlying quantity is a count, the cumulative curve is technically a step function, but it is usually shown as a continuous curve by connecting each day's cumulative total. A cumulative curve for many days (more than 40) often looks smooth, so you can describe its shape by using the following descriptive terms:
When the number of new “data measured” is increasing, the cumulative curve is "concave up." In general, a concave-up curve is U-shaped, like this: ∪. Because a cumulative frequency curve is nondecreasing, it looks like the right side of the ∪ symbol.
When the number of new “data measured” is staying the same, the cumulative curve is linear. The slope of the curve indicates the number of new cases.
When the number of new “data measured” is decreasing, the cumulative curve is "concave down." In general, a concave-up curve looks like an upside-down U, like this: ∩. Because a cumulative frequency curve is nondecreasing, a concave-down curve looks like the left side of the ∩ symbol.
