Lecture 3 - experimental design

Question 1

Two broad types of sampling

Answer 1

-mensuration “natural” experiment -manipulation experiments: = ‘real experiment’

Question 2

Mensuration

Answer 2

Involves making measurements on units (e.g., plots) where treatments are not applied.

Question 3

manipulation

Answer 3

Involves making measurements on ecological units to which treatments have been applied. This is what most people think of as an experiment

Question 4

Experimental unit

Answer 4

The smallest division of the experimental material that can receive any of the treatments.

Question 5

Replication

Answer 5

-repetition of the basic experimental unit (EU) -each treatment is applied to more than one EU -Critical for statistical analysis

Question 6

Replicates

Answer 6

-Independent experimental units (note: subsamples are not replicates) - subsamples cannot be elevated to EUs because artificially inflates replication (used to happen.. would inflate until null hypothesis was rejected..) -How many replicates are enough? n great than or equal to 2.. -Statisticians recommend -10 replicates!!! - very demanding.. Usually need to trade the number of replicates against total number of experimental units you can use. If can’t get as many EUs do you keep replicates? treatments? - classic dilemma.

Question 7

Necessity of replication

Answer 7

1) to estimate experimental error = variability (e.g., S2)
2) insurance against the intrusion of chance events in experiments (= ‘noise”)

ex of noise… Insect outbreak in one of the experimental units (e.g., pots) set up in a green house

Question 8

A good experimental design….

Answer 8

a) know what your experimental unit is - to avoid pseudo replication (which would lead to inflation of EU)
b) randomize - removes bias
c) Get as many replicates as possible - for good statistical analysis - determines variability & adds precision
d) Make sure there is a control

Question 9

Experimental designs

Answer 9

1) Completely randomized design 2) Randomized block design 3) Latin-square design 4) Systematic design

Question 10

Completely randomized design

Answer 10

-experimental units are randomly placed -not recommended when replicates are few -not knowing which cage is control etc..

Question 11

Randomized block design

Answer 11

-EU grouped in blocks -Blocks are groupings of homogeneity (why most popular design) -Each block should contain as many experimental units as treatments. Treatments are randomly assigned within the block.. Ex of treatments.. no burn, low intensity, high intensity -this design corrects the possible affect of a single environmental gradient -Perpendicular to gradient

Question 12

Randomized block design (recommended because..)

Answer 12

a) completely randomized designs can lead to all the units with a particular treatment grouped together - this can be disastrous! b) whole block can be lost without compromising the experiment

Question 13

Systematic design

Answer 13

recommended but with caution - bc of possibility of natural pattern..

Question 14

Latin Square

Answer 14

• two gradients are known to exist (moisture and pH) - latin square corrects effects of the two gradients

Question 15

Rules for latin square

Answer 15

*All treatments must be present in each block. *Treatments must not be duplicated in a block. *Same # across as down

Question 16

Pseudoreplicates

Answer 16

-Not independent -Needs to be avoided -Fake replicates… -Consequence of assuming that subsamples are independent of each other and come from independent EU’s

Question 17

Schematic representation of acceptable (A) and unacceptable (B) experimental designs

Answer 17

• Unacceptable -> some form of pseudoreplication

A-2 & A-3 good if lots of treatments

B-1 very bad because everything on one side gets some treatment.. Not random - so are dependents.. Could have contamination due to proximity…

B-3 even worse.. two chambers.. Would be desirable if using volatiles, but concern is if contamination within samples..

B-4 possible they are dependent

B-5 - means single event that is occurring and each individual is a replicate

Question 18

Examples of Pseudoreplication

Answer 18

1) Burn Experiment: A 1000 ha plot of forest is burned and another plot is not burned. You could then sample 15 10-m2 quadrats on each plot - very flawed.. puts you at B-2 bc never had a chance of being burned/not burned.. More replicates the better.
2) Exclosure Experiment: A 1 ha plot of grassland is fenced and an adjacent 1 ha plot is not fenced. You could then sample 50 1-m2 quadrats on each plot.

**Their are no replicates in either of the above manipulation experiments!!! There are only two EUs in each example and each quadrat is a subsample!

*n=1

Question 19

Exclosure studies in BC

Answer 19

-Look over slides with notes!!

Question 20

Solution to pseudoreplication

Answer 20

-Solution to pseudoreplication is to establish more experimental units (= burn plots and exclosures in our examples).

Note: establishment of more experimental plots adds considerable to the effort and cost of the experiment