Please read data through this: library(sleuth3)

data( ex0725)

Decline in Male Births. Display shows the proportion of male births in Denmark, The

Netherlands, Canada, and the United States for a number of years. (Data read from graphs in Davis

et al., "Reduced Ratio of Male to Female Births in Several Industrial Countries," Journal of the

American Medical Association, 279 (1998): 1018-23.) Also shown are the results of least squares

fitting to the simple linear regression of proportion of males on year, separately for each country,

with standard errors of estimated coefficients in parentheses.

(a) With a statistical computer package and the data in the file ex0725, obtain the least squares

fits to the four simple regressions, individually, to confirm the estimates and standard errors

presented.

(b) Obtain the t -statistic for the test that the slopes of the regressions are zero, for each of the

four countries. Is there evidence that the proportion of male births is truly declining?

(c) Explain why the United States can have the largest of the four t -statistics (in absolute value)

even though its slope is only the third largest (in absolute value).

(d) Explain why the standard error of the estimated slope is smaller for the United States than

for Canada, even though the sample size is the same.

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