Statistics practice problems with answers
This page has 15 statistics practice problems with full worked solutions, ordered from easy to hard. They cover descriptive statistics and box plots, probability rules and Bayes' theorem, discrete random variables, the binomial and normal distributions, sampling distributions, confidence intervals, t-tests and z-tests, p-values and error types, and linear regression. Each solution shows the formula, the numbers substituted into it and how to state the result in context.
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Problem 1: Mean, median, mode and the effect of an outlier
Difficulty: Easy
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Problem 2: Sample variance and standard deviation
Difficulty: Easy
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Problem 3: Five-number summary, IQR and outliers
Difficulty: Easy
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Problem 4: Addition rule, conditional probability and independence
Difficulty: Easy
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Problem 5: Bayes' theorem: a positive screening test
Difficulty: Medium
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Problem 6: Expected value and standard deviation of a discrete random variable
Difficulty: Medium
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Problem 7: Binomial probabilities
Difficulty: Medium
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Problem 8: Normal distribution: probabilities and percentiles
Difficulty: Medium
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Problem 9: Sampling distribution of the mean
Difficulty: Medium
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Problem 10: Confidence interval for a mean (t-interval)
Difficulty: Medium
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Problem 11: Confidence interval for a proportion and sample size
Difficulty: Medium
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Problem 12: One-sample t-test for a mean
Difficulty: Medium
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Problem 13: Interpreting p-values and Type I and Type II errors
Difficulty: Medium
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Problem 14: One-proportion z-test
Difficulty: Hard
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Problem 15: Linear regression: slope, prediction, residual and R-squared
Difficulty: Hard
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Frequently asked questions
When should I use a t-test instead of a z-test?
Use t procedures for a population mean when the population standard deviation is unknown and you estimate it with the sample standard deviation, which is almost always the case in practice. The t-distribution has n - 1 degrees of freedom and heavier tails than the normal distribution, so its critical values are larger, especially for small samples. Tests and intervals for a proportion use z, because the standard error is calculated from the proportion itself.
What is the difference between standard deviation and standard error?
The standard deviation measures how spread out individual observations are. The standard error measures how much a statistic, such as the sample mean, varies from one sample to another. For the mean, the standard error equals the standard deviation divided by the square root of the sample size, so it shrinks as the sample grows while the standard deviation does not.
What does a p-value mean?
A p-value is the probability of getting a result at least as extreme as the one observed, assuming the null hypothesis is true. A small p-value means the data would be surprising if the null hypothesis were true. It is not the probability that the null hypothesis is true, and it does not measure how large or important an effect is.
How do I interpret a 95 percent confidence interval?
Say that you are 95 percent confident the population parameter lies within the interval, and state it in the context of the problem. The 95 percent describes the method: if you repeated the sampling many times, about 95 percent of the intervals built this way would contain the true value. Avoid saying there is a 95 percent probability that the parameter is in your particular interval.
Are these problems suitable for AP Statistics or an introductory university course?
Yes. They cover the core of AP Statistics and of a typical first statistics course at university: exploring data, probability, random variables, the binomial and normal distributions, sampling distributions, confidence intervals, significance tests and linear regression. Chi-square tests and inference for a regression slope are not included here. For a full timed paper, try the free AP Statistics practice exam.
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