Lab 6: Chi-Squared

1. The two species seem to avoid one another: There is a signficant association between them (negative).
2. Null hypothesis: There is no significant association between Avena and Salvia.

11. Calculated Chi-squared: 10.980
12. Critical Chi-squared found along the row for 1 degree of freedom and alpha = 0.05 is 3.841.
13. The X²_calc is, therefore, greater than the X²_crit.
14. Reject the null hypothesis (which makes the alternate hypothesis, that the two species are associated [negatively] look that much better).
15. Prob-value for a X²_calc of 10.980 is ~0.0009, using 11.0 as the way into that table (there's less than one tenth percent probability that you'd get a X² that high if there was nothing but a random process distributing the two species across space).
16. Yule's Q = -0.763

18. There is a significant and strong negative association between Salvia and Avena in this study area. The two species repel one another: Where you find Salvia, you don't find Avena. (Actually, Salvia species engage in allelopathy, chemical warfare -- the volatile oils that make sage smell so good actually kill Avena and several other species' seedlings that have the nerve to germinate in their root zone. And here you thought plants were inert things that just sat there and looked or smelled good <G>!!!)

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First placed on the web: 11/15/99
Last Updated: 06/25/07