A random sample of n₁ = 10 winter days in Denver gave a sample mean pollution index x₁ = 43. Previous studies show that σ₁ = 23. For Englewood (a suburb of Denver), a random sample of n₂ = 14 winter days gave a sample mean pollution index of x₂ = 34. Previous studies show that σ₂ = 11. Assume the pollution index is normally distributed in both Englewood and Denver. Do these data indicate that the mean population pollution index of Englewood is different (either way) from that of Denver in the winter? Use a 1% level of significance.

What is the value of the sample test statistic? (Test the difference μ₁ − μ₂.)