PUBH220 Biostatistics for Public Health Practice Exam Study Guide
Updated 2026 | Verified Questions and Answers with Detailed Rationales
| Descriptive Statistics, Probability Concepts, Sampling Methods,
Hypothesis Testing, Confidence Intervals, p-Values, Regression Analysis,
Correlation, Data Interpretation, Statistical Software Basics, Public
Health Data Applications | Complete Exam Prep Resource for Public
Health Students Success
Question 1: In a right-skewed distribution of household incomes in a public health
survey, which measure of central tendency is MOST appropriate to report as a
summary statistic?
A. Mean
B. Median
C. Mode
D. Geometric mean
CORRECT ANSWER: B. Median
RATIONALE : In a right-skewed distribution, the mean is pulled toward the tail by
extreme high values, making it unrepresentative of the typical observation. The median,
being the 50th percentile, is resistant to outliers and better reflects the central location
of the majority of the data, which is critical for accurate public health reporting and
policy decisions.
Question 2: A public health researcher calculates a standard deviation of 15 mg/dL
for fasting blood glucose levels in a sample of 200 adults. What does this value
primarily indicate?
A. The average deviation of individual values from the mean
B. The range between the highest and lowest values
C. The precision of the sample mean as an estimate of the population mean
D. The proportion of values falling within one unit of the mean
CORRECT ANSWER: A. The average deviation of individual values from the mean
RATIONALE : Standard deviation quantifies the average amount by which individual
data points deviate from the sample mean. It is a measure of dispersion, not precision
(which relates to standard error) or range. Understanding dispersion is essential in
biostatistics for assessing variability in health outcomes.
Question 3: Which statistic is defined as the difference between the third quartile
(Q3) and the first quartile (Q1) in a dataset?
A. Variance
B. Standard deviation
C. Interquartile range
D. Coefficient of variation
,CORRECT ANSWER: C. Interquartile range
RATIONALE : The interquartile range (IQR) = Q3 − Q1 represents the middle 50% of the
data and is a robust measure of spread unaffected by extreme values. It is frequently
used in public health to describe the distribution of skewed variables like hospital
length of stay or pollutant concentrations.
Question 4: When constructing a boxplot for systolic blood pressure
measurements, which component identifies potential outliers?
A. The median line within the box
B. The whiskers extending to 1.5 × IQR beyond the quartiles
C. The width of the box
D. The mean marked with an "X"
CORRECT ANSWER: B. The whiskers extending to 1.5 × IQR beyond the quartiles
RATIONALE : In standard boxplot methodology, whiskers extend to the most extreme
data points within 1.5 times the interquartile range from the quartiles. Points beyond
this range are plotted individually as potential outliers, aiding in the visual detection of
anomalous health measurements that may warrant further investigation.
Question 5: For a categorical variable representing blood type (A, B, AB, O), which
measure of central tendency is statistically appropriate?
A. Mean
B. Median
C. Mode
D. Standard deviation
CORRECT ANSWER: C. Mode
RATIONALE : Nominal categorical variables like blood type lack numerical order,
making mean and median undefined. The mode—the most frequently occurring
category—is the only valid measure of central tendency for such data, essential for
summarizing demographic characteristics in public health surveillance.
Question 6: If every value in a dataset of body mass index (BMI) scores is multiplied
by 2, how does the variance change?
A. It remains unchanged
B. It is multiplied by 2
C. It is multiplied by 4
D. It is divided by 2
CORRECT ANSWER: C. It is multiplied by 4
RATIONALE : Variance is expressed in squared units; thus, multiplying all data points by
a constant k scales the variance by k². Here, k = 2, so variance increases by a factor of 4.
,This property is fundamental when transforming variables for analysis or
standardization in biostatistical modeling.
Question 7: The coefficient of variation (CV) is particularly useful when comparing
variability across datasets that differ in:
A. Sample size
B. Units of measurement or magnitude of means
C. Distribution shape
D. Number of variables
CORRECT ANSWER: B. Units of measurement or magnitude of means
RATIONALE : CV = (standard deviation / mean) × 100% expresses variability relative to
the mean, enabling comparison of dispersion across variables with different units (e.g.,
weight in kg vs. height in cm) or vastly different means, which is valuable in multi-
variable public health assessments.
Question 8: In a histogram of daily step counts from a wearable device study, a long
tail to the right indicates:
A. Negative skewness
B. Positive skewness
C. Bimodality
D. Uniform distribution
CORRECT ANSWER: B. Positive skewness
RATIONALE : Positive (right) skewness occurs when the tail extends toward higher
values, indicating that most observations are clustered at lower values with fewer
extreme high values. Recognizing skewness guides the choice of statistical methods
and transformations in public health data analysis.
Question 9: A z-score of −1.5 for a child's weight-for-age percentile indicates that
the child's weight is:
A. 1.5 standard deviations above the mean
B. 1.5 standard deviations below the mean
C. At the 15th percentile
D. 1.5 times the mean weight
CORRECT ANSWER: B. 1.5 standard deviations below the mean
RATIONALE : A z-score quantifies how many standard deviations an observation lies
from the mean. A negative value indicates a position below the mean. This
standardization is critical in pediatric growth monitoring and epidemiological
comparisons across populations.
Question 10: Which transformation is MOST appropriate to reduce right-skewness
in a variable like serum triglyceride levels before parametric analysis?
, A. Square root transformation
B. Logarithmic transformation
C. Reciprocal transformation
D. No transformation needed
CORRECT ANSWER: B. Logarithmic transformation
RATIONALE : Logarithmic transformation compresses large values more than small
ones, effectively reducing positive skewness and stabilizing variance. This is commonly
applied to biological variables with right-skewed distributions to meet normality
assumptions for t-tests or regression.
Question 11: If the probability of exposure to a contaminated water source is 0.2
and the probability of developing gastrointestinal illness given exposure is 0.3,
what is the joint probability of both events occurring assuming independence?
A. 0.06
B. 0.20
C. 0.30
D. 0.50
CORRECT ANSWER: A. 0.06
RATIONALE : For independent events, P(A and B) = P(A) × P(B). Thus, 0.2 × 0.3 = 0.06.
Understanding joint probability is foundational for risk assessment and modeling
disease transmission in environmental public health.
Question 12: Which condition is REQUIRED for a random variable to follow a
binomial distribution?
A. Continuous outcomes with constant variance
B. Fixed number of independent trials with two possible outcomes
C. Events occurring at a constant rate over time
D. Normally distributed residuals
CORRECT ANSWER: B. Fixed number of independent trials with two possible
outcomes
RATIONALE : The binomial distribution models the number of successes in n
independent Bernoulli trials, each with success probability p. This is applicable to
public health scenarios like the number of vaccinated individuals who develop
immunity in a fixed sample.
Question 13: In a normal distribution, approximately what percentage of
observations fall within two standard deviations of the mean?
A. 68%
B. 90%
C. 95%
D. 99.7%
Updated 2026 | Verified Questions and Answers with Detailed Rationales
| Descriptive Statistics, Probability Concepts, Sampling Methods,
Hypothesis Testing, Confidence Intervals, p-Values, Regression Analysis,
Correlation, Data Interpretation, Statistical Software Basics, Public
Health Data Applications | Complete Exam Prep Resource for Public
Health Students Success
Question 1: In a right-skewed distribution of household incomes in a public health
survey, which measure of central tendency is MOST appropriate to report as a
summary statistic?
A. Mean
B. Median
C. Mode
D. Geometric mean
CORRECT ANSWER: B. Median
RATIONALE : In a right-skewed distribution, the mean is pulled toward the tail by
extreme high values, making it unrepresentative of the typical observation. The median,
being the 50th percentile, is resistant to outliers and better reflects the central location
of the majority of the data, which is critical for accurate public health reporting and
policy decisions.
Question 2: A public health researcher calculates a standard deviation of 15 mg/dL
for fasting blood glucose levels in a sample of 200 adults. What does this value
primarily indicate?
A. The average deviation of individual values from the mean
B. The range between the highest and lowest values
C. The precision of the sample mean as an estimate of the population mean
D. The proportion of values falling within one unit of the mean
CORRECT ANSWER: A. The average deviation of individual values from the mean
RATIONALE : Standard deviation quantifies the average amount by which individual
data points deviate from the sample mean. It is a measure of dispersion, not precision
(which relates to standard error) or range. Understanding dispersion is essential in
biostatistics for assessing variability in health outcomes.
Question 3: Which statistic is defined as the difference between the third quartile
(Q3) and the first quartile (Q1) in a dataset?
A. Variance
B. Standard deviation
C. Interquartile range
D. Coefficient of variation
,CORRECT ANSWER: C. Interquartile range
RATIONALE : The interquartile range (IQR) = Q3 − Q1 represents the middle 50% of the
data and is a robust measure of spread unaffected by extreme values. It is frequently
used in public health to describe the distribution of skewed variables like hospital
length of stay or pollutant concentrations.
Question 4: When constructing a boxplot for systolic blood pressure
measurements, which component identifies potential outliers?
A. The median line within the box
B. The whiskers extending to 1.5 × IQR beyond the quartiles
C. The width of the box
D. The mean marked with an "X"
CORRECT ANSWER: B. The whiskers extending to 1.5 × IQR beyond the quartiles
RATIONALE : In standard boxplot methodology, whiskers extend to the most extreme
data points within 1.5 times the interquartile range from the quartiles. Points beyond
this range are plotted individually as potential outliers, aiding in the visual detection of
anomalous health measurements that may warrant further investigation.
Question 5: For a categorical variable representing blood type (A, B, AB, O), which
measure of central tendency is statistically appropriate?
A. Mean
B. Median
C. Mode
D. Standard deviation
CORRECT ANSWER: C. Mode
RATIONALE : Nominal categorical variables like blood type lack numerical order,
making mean and median undefined. The mode—the most frequently occurring
category—is the only valid measure of central tendency for such data, essential for
summarizing demographic characteristics in public health surveillance.
Question 6: If every value in a dataset of body mass index (BMI) scores is multiplied
by 2, how does the variance change?
A. It remains unchanged
B. It is multiplied by 2
C. It is multiplied by 4
D. It is divided by 2
CORRECT ANSWER: C. It is multiplied by 4
RATIONALE : Variance is expressed in squared units; thus, multiplying all data points by
a constant k scales the variance by k². Here, k = 2, so variance increases by a factor of 4.
,This property is fundamental when transforming variables for analysis or
standardization in biostatistical modeling.
Question 7: The coefficient of variation (CV) is particularly useful when comparing
variability across datasets that differ in:
A. Sample size
B. Units of measurement or magnitude of means
C. Distribution shape
D. Number of variables
CORRECT ANSWER: B. Units of measurement or magnitude of means
RATIONALE : CV = (standard deviation / mean) × 100% expresses variability relative to
the mean, enabling comparison of dispersion across variables with different units (e.g.,
weight in kg vs. height in cm) or vastly different means, which is valuable in multi-
variable public health assessments.
Question 8: In a histogram of daily step counts from a wearable device study, a long
tail to the right indicates:
A. Negative skewness
B. Positive skewness
C. Bimodality
D. Uniform distribution
CORRECT ANSWER: B. Positive skewness
RATIONALE : Positive (right) skewness occurs when the tail extends toward higher
values, indicating that most observations are clustered at lower values with fewer
extreme high values. Recognizing skewness guides the choice of statistical methods
and transformations in public health data analysis.
Question 9: A z-score of −1.5 for a child's weight-for-age percentile indicates that
the child's weight is:
A. 1.5 standard deviations above the mean
B. 1.5 standard deviations below the mean
C. At the 15th percentile
D. 1.5 times the mean weight
CORRECT ANSWER: B. 1.5 standard deviations below the mean
RATIONALE : A z-score quantifies how many standard deviations an observation lies
from the mean. A negative value indicates a position below the mean. This
standardization is critical in pediatric growth monitoring and epidemiological
comparisons across populations.
Question 10: Which transformation is MOST appropriate to reduce right-skewness
in a variable like serum triglyceride levels before parametric analysis?
, A. Square root transformation
B. Logarithmic transformation
C. Reciprocal transformation
D. No transformation needed
CORRECT ANSWER: B. Logarithmic transformation
RATIONALE : Logarithmic transformation compresses large values more than small
ones, effectively reducing positive skewness and stabilizing variance. This is commonly
applied to biological variables with right-skewed distributions to meet normality
assumptions for t-tests or regression.
Question 11: If the probability of exposure to a contaminated water source is 0.2
and the probability of developing gastrointestinal illness given exposure is 0.3,
what is the joint probability of both events occurring assuming independence?
A. 0.06
B. 0.20
C. 0.30
D. 0.50
CORRECT ANSWER: A. 0.06
RATIONALE : For independent events, P(A and B) = P(A) × P(B). Thus, 0.2 × 0.3 = 0.06.
Understanding joint probability is foundational for risk assessment and modeling
disease transmission in environmental public health.
Question 12: Which condition is REQUIRED for a random variable to follow a
binomial distribution?
A. Continuous outcomes with constant variance
B. Fixed number of independent trials with two possible outcomes
C. Events occurring at a constant rate over time
D. Normally distributed residuals
CORRECT ANSWER: B. Fixed number of independent trials with two possible
outcomes
RATIONALE : The binomial distribution models the number of successes in n
independent Bernoulli trials, each with success probability p. This is applicable to
public health scenarios like the number of vaccinated individuals who develop
immunity in a fixed sample.
Question 13: In a normal distribution, approximately what percentage of
observations fall within two standard deviations of the mean?
A. 68%
B. 90%
C. 95%
D. 99.7%
