Hearing. Vendors of hearing aids test them by having patients listen to lists of words and repeat what they hear. The word lists are supposed to be equally difficult to hear accurately. But the challenge of hearing aids is perception when there is background noise. A researcher investigated four different word lists used in hearing assessment (Loven, 1981). She wanted to know whether the lists were equally difficult to understand in the presence of a noisy background. To find out, she tested 24 subjects with normal hearing and measured the number of words perceived correctly in the presence of background noise. Here are the boxplots of the four lists.

a) Is this an experimental or observational study? Explain.

b) Is this a prospective or retrospective study? Explain.

c) State the null and alternative hypothesis.

d) Calculate the F-statistic.

e) The P-value for this statistic turns out to be 0.002. State the conclusion. Can a causal link be established? Explain.