SOC 221 Statistical Concepts and Methods for the Social Sciences

SOC 221 Statistical Concepts and Methods for the Social Sciences

SOC 221: Statistical Concepts and Methods for the Social Sciences

“Conservationists have despaired over destruction of tropical rain forest by logging, clearing and burning.” These words begin a report on a statistical study of the effects of logging in Borneo. Here are data on the number of tree species in 12 unlogged forest plots and 9 similar plots logged 8 years earlier. The numbers of tree species for a plot in the two groups are shown below.  Using these data and a .05 alpha level, calculate the necessary values (e.g. mean, SD) and test the null hypothesis that unlogged and logged plots do not differ with respect to the number of tree species.

UNLOGGED (Group 1): 22, 18, 22, 20, 15, 21, 13, 13, 19, 13, 19, 15

LOGGED (Group 2):  17, 4, 18, 14, 18, 15, 15, 10, 12

What is the null hypothesis for this test?

What is the alternative hypothesis for this test?

Is this a one-sided or two-sided test?  How do you know?

Will you use a z-test or a t-test in this situation?  Why?

What is the critical value of the test statistic?

What is the value of the test statistic calculated from the sample results?

What decision about the null hypothesis does your test lead to?

Is the observed the difference in tree species statistically significant?  Explain.

Would your decision be different if you used an alpha level of .01?  Explain.

If you increased the size of your sample by five times so that you had 60 unlogged plots and 45 logged plots, but the group means and variances remained the same, would you reach the same conclusion about the statistical significance of the tree species difference in plots?  Report the numbers used to make your decision.

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