FW854 – Assignment #6, 2007: Using decision analysis to design a management
experiment
DUE Monday, December 3, 2007
Use the workbook you explored in class today (Salmon_value_of_AM.xls) to complete
the following tasks. Open the workbook and go to the “Value of Information” page.
1. Find the escapement level that gives the best 20-yr discounted yield, if the
Ricker model is correct. Record the optimum escapement here to the nearest
tenth of a million fish
_____________
2. Find the optimum escapement for the Beverton-Holt model and record it here
_______________
3. Use the optimum escapements for Ricker and Beverton Holt as the decision
alternatives (in cells A15, 16). Leave the probabilities of the alternative models
at 0.5 each. Which is the preferred decision? What is its expected value?
Preferred decision _______________
Expected value
_______________
4. a) Try adjusting the probabilities of the two models (Ricker vs Bev-Holt) until the
preferred decision changes. What is the minimum probability for the Bev –Holt
model at which the preferred decision changes?
Probability of Bev_Holt __________
b) What is the EVPI for this situation? [Note: the value in cell E18 of the
workbook will be incorrect. You need to fix it to get the correct EVPI –
look at the formula in cell D18]
EVPI _______________
5. Now set the probabilities for the two models back to 0.5 each and compute a
new EVPI. It should be smaller than the one you calculated for question number
4. Can you explain why it’s smaller, despite the fact that you are arguably “more
uncertain” and thus have more to gain from perfect knowledge, when you have
essentially no idea which model is correct?
6. Now switch to the “Experiment” page. Assume that the status quo management
strategy is to allow an escapement of 1.6 million salmon. Simulate two
management experiments and report the results of each experiment below.
Experiment #
Escapement
Duration
EVII
Expmt Cost
Which experiment is “better”, from an expected value point of view?
7. Increase the recruitment variability for both models (parameter “s” in cells A6
and A10 on the Experiment page) from 0.3 to 0.6. Repeat one of your
experiments, and discuss any differences this change made to the outcome.
8. Select one of your experiments (one that is “worth doing”), and modify the EVII
and Experiment Cost calculations to reflect different prior probabilities for the
two models. How much greater does the prior probability of the Ricker model
being true have to be, before the experiment is no longer worth doing. Explain
your answer.