Mare 350 Marine Monitoring

Experimental design lecture

Comments:

None so far!!

Laws of experimental design:

• Be able to concisely state to someone else the question you are asking. Your results will only be as coherent and comprehensible as your initial conception of the problem.
• Clearly define the statistical population to sample. Define the statistical population precisely on a local scale, draw statistical conclusions about it, then extrapolate to general conclusions about biological population.
• Define your sampling unit. The sampling unit is the smallest replicated element in your sampling design (e.g., a quadrat, a transect, or a plankton tow).
• Decide where to select a sample: consider the costs and benefits of random, systematic, and stratified sampling methods.
• Decide how often to collect samples: the frequency of sampling will vary in proportion to how fast the system changes. More dynamic systems require more frequent samples.
• Take replicate samples within each combination of time, location, and any other controlled variable.
• Take an equal number of replicate samples for each combination of controlled variables. Calulate sample size based on power analysis.
• To test whether a condition has an effect, collect samples both where the condition is present and where the condition is absent but where all else is the same. An effect can only be demonstrated by comparison to a control.
• Carry out some preliminary sampling to provide a basis for evaluation of sampling design and statistical analysis options. Those that skip this step because they do not have enough time usually end up losing time.
• Verify that your sampling unit is appropriate to the size, densities, and spatial distributions of the organisms you are sampling.

List of references:

Green, R. H. 1979. Sampling design and statistical methods for environmental biologists. John Wiley & Sons, New York. 257 pp.

Hurlbert, S. H. 1984. Pseudoreplication and the design of ecological field experiments. Ecol. Mongr. 54(2): 187-211.

Stewart-Oaten, A. and W. W. Murdoch. 1986. Environmental impact assessment: "pseudoreplication" in time? Ecology 67(4): 929-940.

Stewart-Oaten, A., J. R. Bence and C. W. Osenberg. 1992. Assessing effects of unreplicated perturbations: no simple solutions. Ecology 73(4): 1396-1404.

Toft, C. A. and P. J. Shea. 1983. Detecting community-wide patterns: estimating power strengthens statistical inference. Amer. Nat. 122(5): 618-625.

Underwood, A. J. 1981. Techniques of analysis of variance in experimental marine biology and ecology. Oceanogr. Mar. Biol. Ann. Rev. 19: 513-605.

Winer, B. J. 1971. Statistical principles in experimental design. 2nd ed. McGraw-Hill, New York. 907 pp.