Management&BACI07A

Management&BACI07A - Impact Assessment ESM 206A 21...

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Impact Assessment ESM 206A 21 November 2007
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Impact assessment What is the impact of one or more management techniques on an environmental variable of interest? Effects of grazing on biodiversity in California grasslands Establishment of marine protected areas in Channel Islands – how do they affect lobster abundance? Has some “event” caused a change in the environment? Power plant goes on line Hurricane Challenge is to establish causation This is a conceptual issue, not a statistical issue!
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San Onofre Nuclear Power Plant Cooling water discharge creates turbidity due to volume of flow This has potential negative effects on giant kelp: Reduced light at sea floor reduces growth and survival of baby kelp Sediments bury hard substrate Data collected on total area of kelp forest, using sidescan sonar Question: did the power plants negatively impact the kelp bed adjacent to the cooling water discharge site?
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Before-After (BA) Suppose we have data from power plant outflow site before and after operation began Is kelp area lower after the power plant went online? Impact site 0 20 40 60 80 100 120 140 Kelp area (hectares) Before After 38 . 48 57 . 100 after before = = x x
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Two-sample t-test If two samples, X and Y, are from populations with the same mean Then the quantity follows a t distribution with n x + n y – 2 degrees of freedom Y X H μ = : 0 2 2 , where ( 1) ( 1) 1 1 ( 1) ( 1) x y x x y y x y x y x y x y t s s n s n s n n n n - - - = - + - = + - + -
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Assumptions of 2-sample t-test Populations from which x and y are sampled are normally distributed Test is pretty robust as long as sample sizes are similar and 2-tailed tests are being considered The more samples, the better If non-normality is strong, don’t put confidence in alpha levels below 0.01 Populations from which x and y are sampled have same variances Violations mean that P values will be somewhat too small Use Welch’s approximate t test instead
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Welch’s approximate t-test If two samples, X and Y, are from populations with the same mean but different variances Then the quantity follows a t distribution with degrees of freedom 2 2 y x x y x y t s s n n - = + 2 2 2 2 2 2 2 1 1 y x x y y x y x x y s s n n df s s n n n n + = + - -
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BA (continued) t-Test: Two-Sample Assuming Equal Variances Before After Mean 100.5714 48.38462 Variance 852.5714 503.7564 Observations 14 13 Pooled Variance 685.1402 Hypothesized Mean Difference 0 df 25 t Stat 5.176365 P(T<=t) one-tail 1.18E-05 t Critical one-tail 1.708141 P(T<=t) two-tail 2.36E-05 t Critical two-tail 2.059539
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This note was uploaded on 08/06/2008 for the course ESM 206 taught by Professor Kendall,berkley during the Spring '08 term at UCSB.

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Management&amp;BACI07A - Impact Assessment ESM 206A 21...

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