Alarming levels of mercury are starting to creep into our food chain: Our fi sh are showing up with levels of mercury in them that used to be absent. Are the mercury levels the same in all fish? The data set Mercury gives mercury levels in parts per million for four types of fish.

a. What is the explanatory variable? What is the response variable?
b. What are the observational units for this data set?

c. Are the validity conditions met to carry out a theory-based test? Explain.
d. Carry out the appropriate test of significance. Follow up with 95% confidence intervals if the result is significant to fi nd where differences are.
e. Write, as if for a cnn.com health blog, a paragraph summarizing your findings.