Why Biologists Need Sampling,
Experimental Design, and Statistics
Based on Chapter 1 of D. Heath (1995).
An Introduction to Experimental Design
and Statistics for Biology. CRC Press.
What do biologists “do”?
• Biologists study the living world through
observation and experimentation
• Two types of scientific questions:
1. Description (“WHAT?” questions)
2. Explanation (“HOW?” questions)
QUESTION:
What is the mass of a M & M ?
In-Class Exercise
• 1 person from each group will come up and
choose 10 M &M from a jar
• Each group will then use a balance to measure
the mass of each of these 10 beans
• Finally, each group will characterize the mass
of the M & M in the jar
QUESTION:
What is the mass of a M & M?
• We could say the “true value” is: 1.63 g
• Is it correct to say this?
• NO, because there is always variation
associated with our measurements!
VARIABILITY
• Experimental variation: variability introduced by
the observer/experimenter or by the techniques
and equipment used
• Organismal variation: natural variability in
genotypes and phenotypes
• Spatiotemporal variation: variability in the
spatial and/or temporal distribution of organisms,
ecosystems, etc.
• PROBLEM: all of this variability gets in the way of
(or should be a part of?) our attempts to describe
and explain the living world!
How do we deal with variability?
• One approach is to measure every single
entity in every possible location of our
population of interest (NOTE: this may not
always be a “biological population”)
• BUT, this is rarely possible in practice!
• Potential solution: we measure a subset of all
the possible entities (i.e., a sample) and use
these values to make inferences about the
population
Sampling and Experimental Design
• We want our sample to be representative of all
the possible observations in the population
• Potential problems:
1. Bias: when certain observations occur more or less
frequently in the sample than they do in the
population
2. Sampling variation: chance variation in the makeup
of a sample (aka, “sampling error”)
• We can use the principles of experimental
design to develop a sampling strategy that
minimizes these problems
Sampling and Statistics
We can assume that there is a “true value”,
however:
• Even with an effective sampling strategy, it will
never be possible to obtain the “true”
population value (due to sampling variation)
• BUT, we can use statistical analysis to get a
measure of how close our sample estimate is
to the “true” value
Beyond Description: Explanation
• Describing biological patterns is important,
but we also want to understand the processes
that generated the observed patterns
• Explanation: involves identifying the factors or
causes of what has been described
• PROBLEM: the complexity and variability of
biological systems often means there are
many possible factors involved
EXAMPLE: Predator-Prey Relationships
Predator
density
Prey
density
Environment
Observational Studies
(aka, “Natural Experiments”)
• After making hypotheses about potential
factors, we can take advantage of naturally
occurring variation in the factors to test for
differences in the association among the
variable of interest
• Common in ecology
• Problem: such studies cannot determine the
direction of cause-and-effect (i.e., causal
connections)
Manipulative Experiments
• The investigator sets up the experiment so
that the treatments differ only in the factor(s)
of interest
• Can be laboratory, greenhouse, or field
experiments
• Because we reduce possible confounding
factors, we have much more confidence in the
causal relationship between the factor(s) and
the response variable
Experimental vs. Observational Studies
• While manipulative experiments are the most
effective method of establishing causal
relationships, they have some drawbacks:
– Focus on only a few factors
– Experimental manipulation of factors may not be
representative of “natural” conditions
– Often limited to relatively small spatiotemporal
scales
– May not be feasible due to legal, ethical, and/or
practical considerations
Summary
• The starting point of all studies should be one
or more well-defined research questions
• Sampling, experimental design, and statistical
analysis are crucial to almost all biological
studies
• It is essential to consider these three issues
together and, ideally, before any of the
practical work of the study is carried out
For next class:
•
•
•
Read Chapter 6 of Gotelli and Ellison (2004)
Read Hurlbert (1984 )
Prepare a brief written summary (1 page) on an
example of experimental design including:
–
–
•
Your research question and hypothesis
How you plan to eliminate alternative explanations and control for
confounding factors as you evaluate your hypothesis
Homework 1 is due August 31:
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Read Ehrlen 1992 and Harvell 1992 and answer the following questions
How these studies were pseudo-replicated?
How did this affect the information he presents?