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The scientific method
Developing hypotheses
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What exactly is a scientific hypothesis?
Science: [scientia, knowledge]
1. Knowledge gained through experience…
2. Accumulated and accepted knowledge that has been systematized and formulated with reference to the discovery of general truths or the operation of general laws
3. Branch of study that is concerned with observation and classification of facts concerned with the physical world and its phenomena
Science can only state what is, not what should be.
Albert Einstein
Which of the following statements can be tested scientifically?
• Most of the energy coming from the sun is in the form of visible light.
• Unicorns exist.
• Shelley wrote beautiful poetry.
• The Earth was created over four billion years ago.
• Diamond is harder than steel.
• Diamonds are more beautiful than rubies.
• The claim is sometimes made that the scientific method produces closer and closer approximations to "reality (truth)." • Is this a scientific statement? Why or why not? The Scientific Method
Scientists develop their knowledge by observation and experimentation Observation is used in two ways:
1. Inductive reasoning ‐ discovering general principles by the careful examination of specific cases. Here the scientist organizes data (facts) into categories and asks what they have in common
2. Deductive reasoning starts with general cases and proceeds to specific cases ‐ it makes relationships clearer and allows predictions to be made Scientific knowledge advances by a method known as "strong inference" Strong inference works as follows:
1. Make an observation or measurement
2. State an hypothesis 3. Test the hypothesis 4. Publish the results
5. Restate the hypothesis, test again
Observation
Semmelweis (1856), observed in hospitals in
Vienna, that 5X the number of women died during
childbirth if they were attended by a physician as
compared to being attended by a nurse
(“childbed fever”).
One of his colleagues died of childbed fever after
cutting his hand with a scalpel during an
autopsy.
Doctors are often doing autopsies before
attending to childbirths.
Observation
(inductive reasoning)
Hypothesis
Semmelweis hypothesized that “cadaveric matter” was
transmitted from the autopsy room to the delivery room.
Observation
(inductive reasoning)
Hypothesis
(deductive reasoning)
Prediction
Washing hands would eliminate the cadaveric matter
and reduce childbed fever.
null hypothesis vs. alternative hypothesis
Observation
(inductive reasoning)
Hypothesis
(deductive reasoning)
Prediction
Experiment
Observation
Experimental design
• controls
• dependent variable
(what’s affected)
• independent variable
(what’s the “cause”)
• predicted data (must be able to
differentiate between null and
alternative hypotheses)
correlation ≠ causation
correlation ≠ causation
Observation
(inductive reasoning)
Hypothesis
(deductive reasoning)
Prediction
[Experiment]
Experiment
Observation
Analysis
(Observation = Prediction?)
Childbed fever deaths absent when physicians washed their hands
Observation
(inductive reasoning)
Hypothesis
(Deductive
Reasoning)
Prediction
[Experiment]
Experiment
Observation
Analysis
(Observation = Prediction?)
Conclusion
Transmission of
cadaveric matter causes
childbed fever
Observation
(inductive reasoning)
Hypothesis
(deductive reasoning)
Prediction
[Experiment]
Experiment
Observation
Analysis
(Observation = Prediction?)
Conclusion
What other hypotheses
could be made from the
conclusion?
Observation
(inductive reasoning)
Hypothesis
(deductive reasoning)
Prediction
[Experiment]
Experiment
Observation
Analysis
(Observation = Prediction?)
Conclusion
What would you
predict from your
hypothesis?
How would you
test it experimentally??
Observation
(inductive reasoning)
Hypothesis
(Deductive
Reasoning)
Prediction
[Experiment]
Experiment
Observation
Analysis
(Observation = Prediction?)
Conclusion
Theory
Law
Fact
Preconceptions can influence scientific method
Identifying
Patterns
Observations
Experimentation
Data
Preconceptions
Hypothesis
Prediction
”vapors”, spontaneous generation, women as “weak”
accumulated
scientific data
Theories in science
build from the
accumulation of multiple
investigative efforts
formulation of
hypothesis
observation and
experimentation
new data
conclusions
1
2
3
theory
etc
Summary
• The scientific method is in essence a process of observing natural phenomena which leads to the asking of questions about those phenomena which leads to the offering of explanations that can subsequently be tested. Explanations can be of three different types of claims:
1. Causal mechanisms – cause: men who take a single buffered aspirin each day have a 50% lower chance of heart attack than men who do not take aspirin.
2. Laws – When heat is applied to a container of gas, the pressure increases. Why? Guy‐
Lussac’s Law – If volume is held constant, the pressure exerted by a gas will vary directly with the temperature.
3.
Underlying processes ‐
A Tungsten bulb is only 10% efficient, but a fluorescent bulb is 90% efficient.
Tungsten filament is heated until it glows – 90% of the energy is lost as heat.
Mercury vapor in an enclosed tube is energized by electrons, causing it to be absorbed by a phosphorescent coating, causing the coating to fluoresce – only 10% of the energy lost as heat.
Before beginning to test an explanation....
1. Is there an accurate description of the phenomenon to be explained?
2. Are more plausible rival explanations available?
If so – which one do we favor?
Parsimony ‐ principle
Given competing explanations – any of which would, if true, explain a given puzzle – we should initially opt for the explanation that itself contains the least number of puzzling notions.
„Ockham´s razor“ Wilhelm von Ockham 1285‐1349
30
Types of hypotheses:
• Conjectural hypotheses: something that is assumed based on reasonable evidence, but that offers nothing testable.
• Explanatory hypotheses: a proposed explanation that needs to be tested.
Types of hypotheses:
• Conjectural hypotheses: something that is assumed based on reasonable evidence, but that offers nothing testable. "I believe that there is intelligent life somewhere in the universe other than earth."
• Explanatory hypotheses: a proposed explanation that needs to be tested. "Salmon swim up streams to breed so that predation on their young is reduced."
How to test an explanatory hypothesis
• We must devise a set of experimental conditions under which something specific will occur if the hypothesis is correct but will not occur if the hypothesis is incorrect.
• Therefore our test must meet two exacting criteria:
– 1. It must predict what will happen if the explanation is correct
– 2. It must predict what will not happen if the explanation is wrong.
Francesco Redi
He who experiments increases knowledge. He who speculates piles error upon error. – Arabic epigraph quoted by Redi
Spontaneous generation of vermin?
Redi’s experiment: open vs. sealed jars (done in 1668).
What have you shown when your
experiment yields the predicted
results?
Can you really show that
something is true?
Popper’s Falsificationism
Sir Karl Popper
1902 - 1994
•
•
•
Once proposed, speculative theories are to be rigorously and ruthlessly tested by observation and experiment Theories that conflict with empirical evidence must be eliminated (falsified) and replaced by new theories
What does it mean when theories do not conflict ?
Popper’s Falsificationism
• Not verification, but falsification
– Reason: the classical problem of induction
•
A theory can never be said to be true
– Of a theory it can only be said that it is the best available in the sense that is better than anything that has come before – at least for the time being. – Conflicting evidence: Elimination of theory
•
Consequence
– there is no certainty in science
– scientific knowledge is always tentative. How do you come up with a good question for your PhD? Including testable hypothesis?
Ant‐Plant ‐ Symbioses:
•
•
Plants: Nesting space and nutrition extrafloral nectaries
foodbodies
Ants: Defence against herbivores and vines / other plants
B. Fiala
The same ant species (of ~10) are found
again and again on the same species of
host plant (~30 species). This is very
interesting – why don´t you look at that?
B. Fiala
Influence of host‐identity vs. host‐distribution on the radiation of obligately plant‐associated ants
Heike Feldhaar
Lehrstuhl für Verhaltensphysiologie und Soziobiologie (Zoologie II)
Universität Würzburg
Scientific work
How do you find an interesting
research question?
• Apply for a PhD and do what you are told…
Scientific work
How do you find an interesting
research question?
• Observation
• Discussion – communication with other people
• Reading!
• Reading!
• Reading!
How do you find the
relevant literature?
How do you find relevant
literature?
• Search engines
–
–
http://scholar.google.de
http://www.ncbi.nlm.nih.gov/sites
–
http://apps.webofknowledge.com/
Wikipedia !!!
Searching for relevant literature
• Search for articles using keywords (topic)
• Look at the abstract – is it really relevant for your work? Only then read a bit further…
• Look at the references – anything cited in an interesting article that may be interesting too?
• You are new to a topic: search for and read (recent but) well cited reviews – screen citing recent articles
Exercise: Tell us what you work on!
What is your project?
What are your hypotheses?
Scientific work
How do you find an interesting
research question?
• Topic
• Why is this interesting to you?
• Why does it matter?
(Interesting to anybody else?
• The answer to the question “What do you work on?” is not “I work on species x” or “I work on interactions between x and y”.
> Can you clearly state your research question?
• “Because it is poorly known” is not an adequate reason for choosing a dissertation project.
> Why is it a fundamental question?
Hypotheses
State your working hypothesis: Predicted difference
– Good examples (testable!)
• Elefants and rhinos differ in adult weight
• Men have shorter lifespans than women
• Male birds singing more have higher fitness
 A difference can be measured, albeit its tendency is not known beforehand
– Bad examples
• I want to study the adult weight of mammals
• Drosophila show an interesting feeding behaviour because they move irregularly.