rouse@iastate.eduapplying science to fuel and feed our global society jim rouse scc-33 / ucta...

Rouse@iastate.edu Applying science to fuel and feed our global society

Jim RouseSCC-33 / UCTA

1/27/2010

Alpha dogs:The significance of the LSD

Rouse@iastate.edu515-294-5604

Rouse@iastate.edu Applying science to fuel and feed our global society

Today’s topics

• Alpha levels—survey results• Perceptions of alpha• Impact on LSD• Costs of decision errors

Rouse@iastate.edu Applying science to fuel and feed our global society

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

locsreps

ErrorMeanSquaretLSD

*

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)

Rouse@iastate.edu Applying science to fuel and feed our global society

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