Carbon monoxide. The data in Display 9.108 give the “yield†of carbon monoxide (CO) for side stream smoke (what the smoker doesn’t inhale) and mainstream smoke (what the smoker inhales) from eight brands of Canadian cigarettes.
a. Do you have paired data or independent samples? How can you tell?
b. What is your impression from looking at the data: Do you need formal inference to decide whether carbon monoxide levels are higher in side stream smoke? Predict whether the P-value for a two-sided test will be closest to 0.25, 0.10, 0.05, 0.01, or 0.001.
c. To what extent is a probability model for the data justified? What is the basis of the justification? What kinds of inferences are justified: From a sample to a population? About cause and effect? Something else?
 d. Make a scatterplot of the data and describe the pattern in words. Which brands, if any, stand out? Does the shape of the plot suggest that your decision in part a will have a big influence on the P-value?
e. Compute the differences (Cosied − Comin) for the eight brands and show them in a stem plot. Describe the shape of the distribution, and tell whether the shape raises concerns about the accuracy of P-values based on the normal distribution.
 f. Carry out both the paired and the two-sample t-tests. Are your results consistent with your prediction in part d based on the shape of the scatterplot? How close was your prediction about the P-value in part b?
g. Write two or three sentences giving your conclusion about carbon monoxide levels in mainstream and side stream smoke.
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