第3章_实验设计

Balancing and Blocking
There are four general ways to increase precision in any statistical comparison: • 1) Use more homogenous experimental units. This advice is useful for laboratory experimenters but is difficult to use in many field experiments. • 2) Use information provided by related variables that can be measured in each experimental unit. The analysis of covariance is the simplest example of this approach. • 3) Use more replicates. • 4) Use a more efficient experimental design.
• Hurlbert reported that in two separate surveys 26% and 48% of the ecological papers surveyed showed the statistical error of pseudoreplication. Underwood (1981) found statistical errors in 78% of the papers he surveyed in marine ecology. Clearly we need to improve the statistical design of ecological studies. • ------Charles ------Charles J. Krebs,1999,Ecological methodology.
• 我们用t检验比较两组无脊椎动物丰富度的均数： 我们用t
• 用错误重复抽样方法来研究火烧对土壤无脊椎动物的影响。在每 用错误重复抽样方法来研究火烧对土壤无脊椎动物的影响。 块土地重复抽取了5个土壤样本。 块土地重复抽取了5个土壤样本。
• 在这个例子中，应用的统计检验方法是正确的。问题 是，重复抽取土壤样本不适合检验火灾的影响。它只 能衡量同一场火灾造成的影响。两块样地固有的差异 也会对无脊椎动物造成影响。通过这个设计，我们无 法得出关于火灾影响的任何结论，我们只能得出这两 块样地是不同的结论。 • 如果每种处理只有一个实验对象，特别当实验周期比 较短时，我们可以通过不同时间多次重复实验，用实 验组与对照组互换的方法，达到重复的目的。 ------Gerry ------Gerry P. Quinn and Michael J. Keough 2002 Experimental Design and Data Analysis for Biologists. University Press, Cambridge
-----S. -----S. H. Hurlbert, Pseudoreplication and the design of ecological field experiments. 1984, Ecological Monographs
• I strongly disagree with the suggestion(Cox 1958;Cochran and Cox 1957) that the completely randomized design may be most appropriate in “small experiments” ------S.Hurlbert ------S.Hurlbert 1984 • In the studies interspersion is more important than randomization because it deals with the critical question of how the experimental units should be distributed in space. -----C.Krebs 1999 -----C.Krebs
预将15个样品随机分为 组 每组 每组5个 预将 个样品随机分为3组,每组 个. 个样品随机分为 样品编号: 样品编号 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 随机数字: 随机数字 90 96 23 70 00 39 00 03 06 90 55 85 78 38 36 94 37 30 69 余数: 除3余数 0 0 2 1 0 0 0 0 0 0 1 1 0 2 0 余数 组 别: C C B A C C C C C C A A C B C 结 果: A组3个,B组2个,C组10个. 组 个 组 个 组 个 须将C组样品调给 组和B组 方法如下: 组样品调给A组和 须将 组样品调给 组和 组. 方法如下 继续抄随机数字94, 94/10(C组有 个样品 余4, 组有10个样品 继续抄随机数字 组有 个样品)余 组的第4个样品即 号样品给A组 将C组的第 个样品即 号样品给 组. 组的第 个样品即6号样品给 组还剩9个样品 余 则C组还剩 个样品 再抄随机数字 组还剩 个样品. 再抄随机数字37, 37/9余1, 组的第1个样品即 号样品给A组 组还剩8个样品 余此类推, 将C组的第 个样品即 号样品给 组. 则C组还剩 个样品 余此类推 组的第 个样品即1号样品给 组还剩 个样品.余此类推 直到各组样品数相同. 直到各组样品数相同 号样品给B组是否可以 问 题: 1. 1号样品给 组是否可以 号样品给 组是否可以? 2. 样品从 组调到 组及 组的原理是什么 样品从C组调到 组及B组的原理是什么 组调到A组及 组的原理是什么?
第3章 实验设计
1. 实验设计的一般原则 2. 实验设计的类型
There are five components to an experiment：
•Hypothesis •Experimental design •Experimental execution •Statistical analysis •Interpretation
• 目前有一本不错的参考书，总结了当研究地点只 有一个的时候，研究人类活动可能带来的影响时 可以采用的方法。这类实验设计通常称为前后控 制影响（BACI）设计（Green1979，Stewart制影响（BACI）设计（Green1979，StewartOaten等1986），它提供了多种建议，包括在不 Oaten等1986），它提供了多种建议，包括在不 同时间采样，在不同对照区域采样等等。这些设 计都存在争议，对它们正反两方面的评论参见 Keough和Mapstone（1995），Downs等（2002）。 Keough和Mapstone（1995），Downs等（2002）。
General principles of Experimental Design • Randomization • Replication • Balancing and Blocking
Randomization
• A historical perspective:
The notion of randomization was Fisher’s “great contribution to the scientific method”. Yet W. S. Gossett (“Student “), his mentor and friend, and one of the other giants in the history of statistics,guments in favor of strict randomization. Worse yet, Gossett argued that systematic designs were superior.
Replication
Pseudoreplication may be defined, in analysis of variance terminology, as the testing for treatment effects with an error term inappropriate to the hypothesis being considered. (S. Huzlhezt, 1984)
The statistical procedures have to be built in BEFORE the experiment is done, and should not simply be regarded as the numbernumber-crunching that is carried out after the data have already been gathered.
The simplest and most common type of pseudoreplication occurs when there is only one replicate per treatment. For example, there may be one large burned area and one unburned area. If several 1 m2 plots are measured within each area, these 1 m2 plots are not replicates (they are subsamples), and they should not be used in a t-test to compare burned vs. unburned area in general. A t-test would only answer the specific question of this particular burned area differed from this particular unburned area. (Ecological Methodology, 1998)