An economist wants to determine whether the monthly energy cost for families has changed from the previous year, when the mean cost per month was $200. The economist randomly samples 25 families and records their energy costs for the current year.

The economist performs a 1-sample t-test to determine whether the monthly energy cost differs from $200.

  1. Open the sample data, FamilyEnergyCost.MTW.
  2. Choose Stat > Basic Statistics > 1-Sample t.
  3. From the drop-down list, select One or more samples, each in a column and enter Energy Cost.
  4. Select Perform hypothesis test.
  5. In Hypothesized mean, enter 200.
  6. Click OK.

Interpret the results

The null hypothesis states that the mean of the energy costs is $200. Because the p-value is 0.000, which is less than the significance level of 0.05, the economist rejects the null hypothesis and concludes that the average monthly energy cost for families differs from $200. The 95% CI indicates that the population mean is likely to be greater than $200.

One-Sample T: Energy Cost

Descriptive Statistics N Mean StDev SE Mean 95% CI for μ 25 330.6 154.2 30.8 (266.9, 394.2) μ: population mean of Energy Cost
Test Null hypothesis H₀: μ = 200 Alternative hypothesis H₁: μ ≠ 200 T-Value P-Value 4.23 0.000
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