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Flashcards in experimental design Deck (11):
1

What is the scientific experimental design layout?

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2

What are the principles of experimental design?

-Observation:

As cockle density increases, size/growth rate appears lower

-Hypothesis:

Ho = cockle density does not affect growth rate

Ha(lternative) = cockle density affects growth rate

-Methods:

Experimental units (U) = 0.25 m2 plots

Treatments (T) = 0.5, 1, 2 & 4  × natural density

Measurement variable = change in shell height after 6 months

-Design:

Arrangement of U in space (layout)

Number of U per T (number of replicates, n)

Measurement sequence

Statistical analysis

3

Why is replication necessary in experimental design?

-Provides estimate of the error

Inherent variability (e.g differences between individual growth rates)

Experimenter induced (e.g ability to measure small differences in shell height)

-Increases precision of treatment means

Increased ability to detect smaller differences among treatment means

-Choosing the number of replicates

A function of the precision required to detect a given difference among treatment means

Calculated from previous data, pilot studies (see Kingsford & Battershill)

Most likely a function of financial/logistical constraints

4

What are the four key elements of experimental design?

Replication

Randomization

Reduction of error

Range of validity

5

Describe the concept of randomization in experimental design

-Provides an unbiased estimate of the experimental error and the treatment mean error

Critical for statistical tests

-For example:

Location of experimental plots on the shore

Allocation of density treatments to experimental plots

Allocation of cockles to treatments plots

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6

Describe the concept of reducing error in experimental design

-Blocking RCB

-Method

Blocks laid out across gradient

Random assignment 1 replicate T block-1

-Criterion

Identifiable source of variation among U due to position (e.g environmental gradient)

-Advantage

Removes source of variability from error term

Increases sensitivity for detecting T effects

Ensures interspersion

-Limitations

If there is no block effect reduced power

Assumes no interaction between block and T

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7

Describe the concept of increasing complexity in experimental design

-Replicate T within blocks

-Tests

Effect of environmental gradient (compare among shore locations)

Effect of density (compare among density Ts)

Interaction between density & environment (determine whether the effect of the environmental gradient is consistent among density Ts)

Analysis – two way rather than one way ANVOA (see next lecture)

8

Describe the concept of reducing error in experimental design

-Initial homogeneity of U

Variability amongst U may add to error reducing ability to detect differences among T

e.g removal of non-experimental fauna from plots

Applicability of the results?

 

-Concomitant measurements

Additional measurements that may help explain variation among replicates within a T

e.g. account for variations in initial cockle shell height (growth rate is often related to size)

9

Describe the concept of range of validity in experimental design

-Fixed factors

Treatment levels are fixed by the experimenter in order to measure a response

e.g  cockle density T were set at 0.5, 1, 2 & 4  × natural density

Scope of inference  = levels you have tested

 

-Random factors

Treatment levels are chosen at random from a range of possible values

e.g measured cockle density in 20 randomly placed quadrats and have randomly chosen 4 for experimental purposes

Scope of inference  = factor as a whole

 

-DO NOT extrapolate beyond the limits of your experiment

10

what are some of the mistakes people make when designing experiments?

1. Many experiments involve manipulations which can introduce artifacts

2. Cages & fences

-Used to exclude/include different trophic groups

-e.g predation & soft sediment community structure

-Alter microhabitat

Affect flow, shade, create structure

-Alter behavior

-Assessment of artifacts

Partial cages

Direct measurements

Routine monitoring

-Procedural affects may obscure/confound treatment affects

-Alternatives

Chemical barriers

Maintain buffer zones

 

3. Transplant experiments

-Often used to determine impact of environmental conditions on organism performance

Standardize for differences among populations by transplanting from one site

-e.g effects of sewage outfall cockle growth

-Must include a procedural control for handling effects

4. Laboratory experiments

-Sham operations

-Placebos

11

What problems in experimental design are there and how to avoid them?

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