Transcript How do we do science?

Scientific Method and
Hypothesis Testing
“Whereas other types of human speculation
are based on mere opinion, science pursues
and sticks to the facts”
Hypothesis and Perception: The Roots of Scientific Method
By Errol E. Harris
The backbone of science
• The process of “doing” science
– Philosophy that deals with nature and origin of knowledge
• What components are involved?
Scientific Method
From: Martin and Bateson
My version of the flow chart
• Martin and Bateson forgot the two most important steps
Reality of doing science
• No scientist has a flow-chart on the wall
• The truth about hypotheses and predictions
OBSERVE
Ask
Collect and analyze data
Preliminary
observations
Write
Up
Interpret
Choose measures
Choose
recording
method
Hypothesize
predict
analyze
design
experiment
How does science REALLY work?
http://undsci.berkeley.edu/article/scienceflowchart
Coalition for the Public Understanding of Science
In a way, science is self-replicating…
Observation: Caddis in lab unpalatable
Question:
Do they have ttx?
Answer:
yes….?
In a way, science is self-replicating…
New Obs:
small = more ttx, large = less?
New Q:
Why?
Answer:
not sure, but one big
difference in behavior
In a way, science is self-replicating…
New Obs:
Fewer caddisflies at top
New Q:
Do female newts take advantage?
Answer:
Yes, females lay eggs at top
In a way, science is self-replicating…
New Obs:
Females lay eggs high
New Q:
Does this translate
into fitness advantage?
Answer:
Yes!
Falsifiability – part 1
• Example of scientific hypothesis
• Example of non-scientific hypothesis
Infinite number of alternative explanations – not falsifiable
Only takes one observation to falsify – true hypothesis
Falsifiability – part 2
• Leads to an ultimate conclusion:
• If we FAIL TO REJECT = data are consistent with the hypothesis and
it is the likely explanation
• Other explanations/hypotheses may prove to be a better explanation
• GOAL: not prove something, but rule out competing hypotheses, and
make a strong inference
"Philosophy of science is about
as useful to scientists as
ornithology is to birds.“
Richard Feynman
American physicist
Hypothesis
Fish Exercise
• Observation: Some
minnows appear to act
strangely when a
predator is nearby
• Question:
• Hypotheses:
– H0
– HA
• Prediction:
• Experimental Test:
“A student of ethology carefully controls all
aspects of an experiment so that the animal
can give only clear and illuminating responses,
whereupon it does as it damn well pleases.”
Harvard First Law of Animal Behavior
Type of Question
http://www.youtube.com/
watch?v=JIrqirr6jBY
Type of Question
• Pair knowledgeable
“tutor” with naïve
• Then test naïve alone
• Do they acquire the
antipredator behavior
Choosing a study organism
• Easily seen in nature
• Readily available
• Does it acclimate well
• Are life-history
characteristics known
• Background information
• Is species practical for
study
• When is it active
Type of Study
• Two general types:
• Can get quantitative “data” from this type of study
• Often lead to experiments
• Causality is sometimes difficult to infer
– Example: lung cancer and cigarette smoking in humans
• Can we assign treatments?
• Observational study → Increased risk of cancer correlated with smoking
• Conclusions:
Type of Study
Where to study?
Experimental Studies
• May be simple or complex
• Simple experiments:
• Tinbergen 1962
– Made “artificial” gull eggs
– Placed broken eggs at different
distances
– Recorded number of eggs
eaten from different nests
Alternative Method
• Compare the dependent variable between
individuals that get manipulated (treated) and
those that don’t (control)
– Compare predation rate for eggs vs no eggs
Confounding Variables
• Major assumption of
experimental studies
• Can not control everything
• Examples
– Egg shells different sizes
– Nests in different positions
– Temperature differences
between newts
– Tests/trials run at different
times
• Goal?
How do deal with these factors?
• Temp or predation slightly
different at different nests
• Individuals tested at
different times
Use of an individual
as own control
• Most powerful control for inherent variability in ethology
is use the animal as its own control
• i.e. use individual in
control and experimental
conditions
• Example: hellbender behavior
• Another way:
A problem with
manipulated treatments
Example 1: Gull egg experiment looking
at manipulated vs unmanipulated nests
Example 2: Do salamanders change
where they lay eggs when insects are
present
Example 3: Does cutting the olfactory
nerve effect the homing ability of fish
compared to controls
Multiple Independent
Variables
• What effect 2+ factors have
on behavior
• Example: body size and
alarm cues
If we had not looked at body size,
what would the graphs look like?
Gall and Mathis 2011, Copeia
Multiple Dependent
Variables
• Never collect data on only 1
dependent variable
• Example:
– Lines crossed
– Time moving
– Number moves
Gall and Mathis 2011, Copeia
Observer Influence
•
•
•
•
•
Acclimate
Hides/blinds
One-way glass
Mirror
Video camera
Quantifying
Behavioral
Data
• Watch, record, and
catalogue behaviors
• Focus on discrete
behaviors
• Assign each behavior to 2
categories
Why not just use a functional
description from the
beginning?
Types of measures
Sampling - who
• Often necessary to
observe groups of animals
• Must decide who to record
(impossible to watch
everyone)
Sampling - when
• Two parts to determining when to
make your observations
Sampling - when
• Two parts to determining when to
make your observations
Once your data are collected
• Construct a visual
representation of your data
• Is this a “true” difference?
• What information do we
need before making an
inference?
Once your data are collected
• To completely answer
whether the difference in
your data are meaningful
need statistics
Statistical Tests
Parametric
Nonparametric
Depend on meeting assumptions
Robust to violations
Transformations can correct departures
Does not rely on assumptions
Converts data to ranks
Less robust than parametric tests
Types of Tests