Jim Rouse SCC-33 / UCTA 1/27/2010
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Transcript of Jim Rouse SCC-33 / UCTA 1/27/2010
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Jim RouseSCC-33 / UCTA
1/27/2010
Alpha dogs:The significance of the LSD
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Today’s topics• Alpha levels—survey results• Perceptions of alpha• Impact on LSD• Costs of decision errors
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Survey: What level do you use?
n=27 (21 Public, 6 industry)
0.05 0.1 0.2 0.2502468101214161820
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Survey comments• Software provides LSD at 0.05 and
0.2. … often calculate my own at 0.1, because …0.05 is too inclusive…0.2 is too exclusive.
• We use 0.1, but will use 0.05 if we really want to select hard.
• Which one is accurate?
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Impact on LSD• Increase in alpha will decrease LSD• 0.05 is indeed more inclusive
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Least significant difference
locsrepsErrorMeanSquaretLSD
*2
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Making Decisions• H0 No differences• HA Differences
H0 is true H0 is false
Accept H0 Correct Type II error
Reject H0 Type I error Correct / T-III
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Making Decisions• Type I error rate = α, typically 0.05• Type II error rate = β, typically
unspecified and unknown
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Making Decisions• α is inversely related to β• Lower prob of Type I error = higher prob
of Type II error• Key: What is appropriate balance?• Based on cost of errors
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• What is the cost of Type I error?– 2 hybrids are “same”, but we think not
• Result: Choose one over another– No difference, no cost
• Grower’s perspective only!
Decision Errors
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• What is the cost of Type II error?– 2 hybrids are “different”, but we think
not• Result: Choose either
– Choose the better one, no cost– Choose the worse one, cost– Half the time, T-II errors cost growers
• Loss depends on true difference– Yield difference between choices
Decision Errors
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Finding Value in the Data• Determine relative cost of errors• Adjust α to minimize losses
– (a la Carmer in Crop Science, 1976)• Difference between reality and
theory?– Many growers use α = 1– Eliminates Type II errors
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That’s all!Questions—Comments—
Discussion
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New truths…“A new scientific truth does not
triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.”
—Max Planck (founder of quantum theory)
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Background“I have great faith in fools; self-confidence my friends call
it.”—Edgar Allan Poe
“Don’t math it all up.”—Glenda Rouse, my wife