Using the IELTS listening data presented in the previous exercise, practice evaluating data using confidence intervals.

a. Compute the 80% confidence interval.

b. How do the conclusion and the confidence interval change as we move from 95% confidence to 80% confidence?

c. Why don’t we talk about having 100% confidence?

Previous exercise

The International English Language Testing System (IELTS) has six modules, one of which assesses listening skills. IELTS researchers reported that a recent mean for everyone who completed this module in one year was 6.00 and the standard deviation was 1.30 (2013). A sample of 63 international students who were accepted to study at the University of Melbourne in Australia had mean IELTS listening scores of 7.087 with a standard deviation of 0.754 (O’Loughlin & Arkoudis, 2009).