Pseudoreplication and Ecology - Columbia University

Pseudoreplication and Ecology - Columbia University

Pseudoreplication and Ecology Dr. James A. Danoff-Burg Columbia University Purposes of Replication Controls for random or stochastic error E.g., untested independent factors may otherwise determine the outcome of the experiment Increases the precision of the test Increases the generalizability of the test If you test across many sites you can safely

generalize to many others Some Definitions Replicate = Sample Maximize these in your experimental design Greatest number possible, given logistical limitations If you are a professional, use a power analysis Subsample = Pseudoreplicate Only true if the subsamples are incorrectly treated as true replicates for statistical analysis Subsamples: useful to increase the accuracy of the data estimate for that replicate A special type of statistical analysis are therefore

possible Pseudoreplication - Defined Incorrect replication Replicating samples, not treatments Replicates are not independent Problem is that it violates a key assumption of statistical analysis: Independence of replicates Increasing precision of studies if independent Approximates truth better if independent Prevalence of

Pseudoreplication 48% of all studies had pseudoreplication (Hurlbert 1984) 71% of studies using ANOVA (a common statistical test) had design errors (Underwood 1981) Particularly acute in studies with logistical problems Rare animals Transportational or financial limitations

Many of ours! Examples Many samples from a single site These are actually subsamples Only a single sample for each treatment condition These are actually replicates, but cannot do statistics on a sample size of one Single samples from a single site, but replicated in time Would be true samples if the experimental question

is time-dependent If not, it is pseudoreplication Pseudoreplication Example Treatment A Treatment B Site 1 Site 3 Site 2

Site 4 Question What is the affect of treatments A & B? Pseudoreplicati on = treating stars of the same color as replicates Replication = include only a single star of each color

Controlling Pseudoreplication I Know your question Question determines whether design includes pseudoreplication Taxonomic level Ecological hierarchy level Clearly define your independent and dependent variables Controlling Pseudoreplication II What constitutes a unit of data?

Plant branch? Individual? Population? Etc.? Identify what is the unit of replication Individual? Population? Community? Site? Replicate accordingly sites are often the level of replication for our projects Randomize your sampling design Helps to decrease sampling errors For Our Class We will frequently use pseudoreplication Limitations on time (only 4 weekends for our work!) Limitations on transportation (only 1 van!)

Limitations on effort (only you!) Consequently We will frequently treat our pseudoreplicates as true replicates However be aware of this and you will be fine when you design more robust research projects in the future

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