Is the number of tornadoes increasing? The Storm Prediction Center of the National Oceanic and Atmospheric Administration maintains listings of tornadoes, floods, and other weather phenomena. Table summarizes the annual number of tornadoes in the United States between 1953 and 2009.
(a) Make a plot of the total number of tornadoes by year. Does a linear trend over years appear reasonable? Are there any outliers or unusual patterns? Explain your answer.
(b) Run the simple linear regression and summarize the results, making sure to construct a 95% confidence interval for the average annual increase in the number of tornadoes.
(c) Obtain the residuals and plot them versus year. Is there anything unusual in the plot?
(d) Are the residuals Normal? Justify your answer.
(e) The number of tornadoes in 2004 is much larger than expected under this linear model. Remove this observation and rerun the simple linear regression. Compare these results with the results in part (b).
