2. CPH Domain 1: Statistics, Visualization, and Surveillance
After this lesson you can
- Compute the mean, median, range and interquartile range and choose the right center for skewed data
- Read a box plot and apply the 68-95-99.7 rule
- Interpret confidence intervals, p-values and Type I and Type II errors
- Explain qualitative trustworthiness, saturation and triangulation
- Compare passive, active, sentinel and syndromic surveillance and their limitations
- Choose the right chart for categories, distributions, trends and geography
The second lesson on Domain 1. It covers descriptive statistics and box plots, confidence intervals and p-values, qualitative results, surveillance systems and registries, choosing the right chart, and why informatics matters in public health.
Questions and answers
Questions a learner would ask about this lesson. Think through your own answer before you open each one.
- When should I report the median instead of the mean?
Show answer
Report the median when the data are skewed or have outliers, such as income or hospital length of stay. The median resists outliers and the mean does not. In the lesson's nine Harbor County ages, one value of 74 pulls the mean up to 40.9 while the median stays at 36.
- How do I read a confidence interval on the exam?
Show answer
A confidence interval is a range of plausible values for the population quantity, and a narrower interval means a more precise estimate. Check whether it includes the null value, which is 1 for a ratio and 0 for a difference. If it includes the null, the result is not statistically significant at that level; if it excludes the null, it is.
- What is the difference between passive and active surveillance?
Show answer
In passive surveillance, providers report cases on their own initiative, which is cheaper and simpler but less complete and slower. In active surveillance, the health department seeks out cases, which costs more but is more complete and more timely. Sentinel and syndromic systems are other types, and neither counts every case.
- How do qualitative studies show that their findings can be trusted?
Show answer
Qualitative work reports trustworthiness in four parts: credibility, transferability, dependability and confirmability, which roughly parallel internal validity, external validity, reliability and objectivity. Strategies include triangulation, where several sources, methods or analysts converge, and member checking, where participants review the findings. Saturation, the point where new data yield no new themes, is the stopping rule for sampling.
- Which chart should I use for which kind of data?
Show answer
Use a bar chart for categories, a histogram for the distribution of one continuous variable, a line graph for trends over time, a scatter plot for association between two continuous variables, and a map for geography. Avoid pie charts with many categories, truncated axes and dual axes, because they mislead.
Practice quiz
- Question 1The ages of nine people are 31, 33, 34, 36, 36, 38, 41, 45 and 74. What are the mean and the median?
- Question 2For those same nine ages, what is the range?
- Question 3On a horizontal box plot, what does the left edge of the box mark?
- Question 4Systolic blood pressure in a clinic sample is roughly normal with a mean of 120 and a standard deviation of 10. About 95 percent of patients fall between:
- Question 5A study doubles its sample size. Which quantity is expected to shrink?
- Question 6A program reports a risk difference with a 95 percent confidence interval of negative 2 to positive 5 per 100. The best interpretation is:
- Question 7A study fails to detect a program effect that truly exists. This is a:
- Question 8A one-time survey of county adults asks whether they currently have asthma. What measure can it produce?
- Question 9A health department widens its case definition for a reportable illness. What is the expected effect on two surveillance attributes?
- Question 10An epidemic curve shows a single sharp peak of cases by date of onset. This pattern suggests:
Saved in your browser only: no account, no server.
CPH Exam Pathway is an independent course produced by Subthesis for educational purposes. It is not affiliated with or endorsed by the National Board of Public Health Examiners (NBPHE), and completing it does not confer certification. Confirm fees, eligibility, and exam rules against the current NBPHE candidate handbook.