What is a p-chart and how is it used in public health monitoring?

• A. A bar chart for age distribution.
• B. A chart showing incineration costs over time.
• C. A control chart for proportions used to monitor the stability of proportions (e.g., percent positive).
• D. A scatter plot of daily temperatures.

Answer: (C)

A p-chart is a control chart designed for tracking proportions in binary data over time. Each point represents the observed proportion of a positive outcome in a subgroup (for example, the percent of tests that come back positive in a weekly sample). The chart has a center line that reflects the average proportion and upper and lower control limits that define the range of expected variation if the process is stable. In public health, this is especially useful for monitoring indicators that are yes/no in nature, such as the proportion of patients testing positive, vaccination coverage in a clinic, or the rate of adherence to a screening protocol. The key idea is to see whether the proportion stays consistent or if there are signals suggesting a shift or special cause that needs investigation.

You compute the proportion for each subgroup, plot it, and compare it to the control limits, which depend on the overall average proportion and the subgroup size. When a point falls outside the limits or when you see nonrandom patterns, it signals that the process may not be in control and further inquiry is warranted. This approach is distinct from a bar chart (which would just show distribution of ages without time-based control limits), a chart of costs over time (continuous measurement not focused on a proportion), or a scatter plot of temperatures (which shows relationship between two continuous variables rather than a time-ordered proportion).