An S-curve chart is a graphical representation of growth over time. The curve was named after the letter S because its shape begins flat, becomes steep and eventually flattens -- resembling the shape of the letter S. In nature, S-curves are frequently observed when describing the growth of living organism. At first the growth rate is slow, then it accelerates and finally plateaus. Project managers use the S-curve to describe the relationship between cumulative costs or man-hours over time.
Mathematicians and project managers employ a common terminology when discussing S-curves. The starting point in an S-curve chart is called the lower asymptote. The part of the process that is the apex or maturity level is called the point of inflexion. The part where the process reaches a plateau is called the upper asymptote.
How to Construct
The S-chart can be constructed manually or with a computer software program such as Minitab, Microsoft Excel, SAS or SPSS. Two variables are needed to construct the chart. Time is always plotted on the horizontal or X-axis while the variable of interest is plotted on the vertical or Y-axis. Examples of variables that can be plotted on an S-chart include inches of growth, population in people, number of man-hours worked, or cost in dollars.
Used for Tracking
Use the S-curve track the progress of a process over time. If a process is known to have an S shape, graphing the process can tell you if the current process is in the start, growth or maturity phases. Common variables that follow an S-curve are man-hours, labor costs and growth patterns. Two common curves used for comparison are the Pearl and the Gompertz.
Used for Comparison
Compare a project's targeted rates to actual rates using an S-curve. In project management, for example, the number of man-hours is outlined in the project charter. A project manager keeps the project on time by comparing actual man-hours used during each phase with baseline man-hours planned. Slippage occurs when the actual hours used differ from baseline hours. Early detection of slippage can help the project manager reallocate resources to avoid having to ask for more time.