MathematicsInferential StatisticsUniversity

Confidence Interval for a Population Mean (t-interval) Calculator

The t-interval provides a range of values calculated from sample data that is likely to contain the true population mean when the population standard deviation is unknown.

Use the free calculatorCheck the variablesOpen the advanced solver
This is the free calculator preview. Advanced walkthroughs stay in the app.
Result
Ready
Margin of Error

Formula first

Overview

This statistical method utilizes the Student's t-distribution to account for the additional uncertainty introduced by estimating the population standard deviation using the sample standard deviation. It is the preferred method for small sample sizes or when the population variance cannot be assumed known, provided the underlying population is approximately normal.

Symbols

Variables

= Sample Mean, = Critical t-value, s = Sample Standard Deviation, n = Sample Size, ME = Margin of Error

Sample Mean
Variable
Critical t-value
Variable
Sample Standard Deviation
Variable
Sample Size
Variable
ME
Margin of Error
Variable
Upper
Upper Bound
Variable
Lower
Lower Bound
Variable

Apply it well

When To Use

When to use: Use this interval when you need to estimate a population mean from a small sample (n < 30) or when the population standard deviation is unknown.

Why it matters: It allows researchers to quantify the reliability of their estimates in real-world scenarios where data is limited and population parameters are inaccessible.

Avoid these traps

Common Mistakes

  • Using the Z-score instead of the T-score when the population standard deviation is unknown.
  • Forgetting to subtract 1 from the sample size when determining degrees of freedom.

One free problem

Practice Problem

A sample of 10 students has a mean study time of 15 hours with a sample standard deviation of 3. Using a t-score of 2.262 for 95% confidence, find the margin of error.

Critical t-value2.262
Sample Standard Deviation3
Sample Size10

Solve for: margin

Hint: Multiply the t-score by the standard error, which is s divided by the square root of n.

The full worked solution stays in the interactive walkthrough.

References

Sources

  1. Moore, D. S., McCabe, G. P., & Craig, B. A. (2017). Introduction to the Practice of Statistics (9th ed.). W. H. Freeman and Company.
  2. OpenStax. (2018). Introductory Statistics. Rice University.
  3. Moore, D. S., McCabe, G. P., & Craig, B. A. (2017). Introduction to the Practice of Statistics.
  4. OpenStax, Introductory Statistics.
  5. Wackerly, D., Mendenhall, W., & Scheaffer, R. L. (2008). Mathematical Statistics with Applications.