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Flashcards in experimental design Deck (11):

What is the scientific experimental design layout?

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What are the principles of experimental design?


As cockle density increases, size/growth rate appears lower


Ho = cockle density does not affect growth rate

Ha(lternative) = cockle density affects growth rate


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


Arrangement of U in space (layout)

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

Measurement sequence

Statistical analysis


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


What are the four key elements of experimental design?



Reduction of error

Range of validity


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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Describe the concept of reducing error in experimental design

-Blocking RCB


Blocks laid out across gradient

Random assignment 1 replicate T block-1


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


Removes source of variability from error term

Increases sensitivity for detecting T effects

Ensures interspersion


If there is no block effect reduced power

Assumes no interaction between block and T

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Describe the concept of increasing complexity in experimental design

-Replicate T within blocks


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)


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)


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


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


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



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

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