Refer to Exercise 49 on page 238 for a description of a study about seeding clouds.
Exercise 49
Researchers wanted to determine whether seeding (treating) isolated cumulus clouds in south Florida with silver iodide smoke increases rainfall. They randomly assigned 26 clouds to be seeded and 26 clouds to not be seeded. The amounts (in acre-feet) of rainfall from the clouds in the two groups are shown in Table 18.

a. Is the study an experiment or an observational study? Explain.
b. What are the explanatory and response variables?
c. What does random assignment mean in this study? How could this be accomplished?
d. Describe the sample and the population.
e. After performing a statistical test, the study concluded that seeding isolated cumulus clouds increases rainfall. Does this mean there is causality or only an association between the two variables? Explain.

Table 18 Rainfall from Seeded and Control Clouds Rainfall acre-feet) Seeded Clouds Control Clouds 16978 119.0 302.8 345.5 411 4.1 95.0 321.2 4.9 31.4 92.4 147.8 21.7 334.1 274.7 115.3 244.3 36.6 7.7 1656.0 430.0115 830.1 6.3 129.6 175 40.6 32.7 978.0 163.0 24.4 1202.6 4.9 81.2 28.6 17.3 29.0 68.5 489.1 703.4 2745.6 200.7 87.0 274.7 198.6 255.0 242.5 118.3 3724 1.0 47.3 26.1