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University of Ottawa - BIO 5901 © Scott Findlay 01/05/2015 6:10 PM 1 Descriptive versus hypothesis- driven science Descriptive science is concerned with the characterization and/or quantification of patterns in nature The main issues: (a) what are the observed patterns; and (b) are they more imagined than real? Hypothesis-driven science is concerned with testing (scientific) hypotheses advanced to explain the (real, one assumes) patterns uncovered by descriptive science. The main issue: how likely is it that the hypothesis in question is true?
University of Ottawa - BIO 5901 © Scott Findlay 01/05/2015 6:10 PM 2 What is hypothesis-driven science? The accumulation of knowledge about the world through the testing of causal theories (explanations). A causal theory is a statement about the cause(s) of observed phenomenon (“events”, “effects”) Science attempts to infer causal relationships (“If A, then B”) by application of the scientific method. Effect (Observed) Cause CausalityInference
University of Ottawa - BIO 5901 © Scott Findlay 01/05/2015 6:10 PM 3 Causal explanations (hypotheses) for observed phenomena Two types of causal explanations: “scientific”, based on application of the scientific method, and “non-scientific” (for example, appeal to received wisdom). But what exactly is a “scientific” hypothesis/theory? Observed phenomena (what we want to explain) Observed phenomena (what we want to explain) Scientific explanations (hypotheses, theories) Scientific explanations (hypotheses, theories) Non-scientific explanations (hypotheses, theories) Non-scientific explanations (hypotheses, theories)
University of Ottawa - BIO 5901 © Scott Findlay 01/05/2015 6:10 PM 4 What makes a “scientific” hypothesis? According to Sir Karl Popper, all scientific hypotheses must be refutable, at least in principle. A refutable hypothesis is one for which, at least in principle, there are empirical observations which could be inconsistent with the hypothesis. So testability = refutability (falsifiability)
University of Ottawa - BIO 5901 © Scott Findlay 01/05/2015 6:10 PM 5 A simple example: why the bathroom light doesn’t work Bulb burnt out Bulb burnt out Short in circuit Power off to house Power off to house Hypotheses Light switch on, but no light Light switch on, but no light What we want to explain
University of Ottawa - BIO 5901 © Scott Findlay 01/05/2015 6:10 PM 6 Hypotheses and predictions Hypothesis: a statement about the cause(s) of some observed event/pattern. Prediction: the empirical result/pattern one will see if the hypothesis is true. Deduction : the logical connection between hypothesis and prediction. … then the light will work if the old bulb is replaced (P). … then the light will work if the old bulb is replaced (P). If the light bulb is burnt out (H)…
University of Ottawa - BIO 5901 © Scott Findlay 01/05/2015 6:10 PM 7 Deduction Deduction: if the axioms (premises) are true, the conclusion is necessarily true (reasoning from general to particular). All swans are white. This bird is a swan. This bird is white. If (this is a swan) then (it is white). This bird is a swan. This bird is white.
University of Ottawa - BIO 5901 © Scott Findlay 01/05/2015 6:10 PM 8 Hypothesis and prediction: the logical structure Hypotheses can NEVER be proven (i.e. shown to be true with unit probability) The process of refutation requires that the relationship between H and P be DEDUCTIVE! If H then P P H H Fallacy of affirming the consequence If H then P - P - H No logical fallacy!
University of Ottawa - BIO 5901 © Scott Findlay 01/05/2015 6:10 PM 9 Hypotheses, experiments and predictions New bulb will blow &/or breaker will trip Replace bulb with new bulb Short in circuit No other switch will work Try other electric switches Power off to house Light will workReplace bulb with new bulb Light bulb burnt out PredictionExperimentHypothesis
University of Ottawa - BIO 5901 © Scott Findlay 01/05/2015 6:10 PM 10 Same hypothesis, different experiment, different prediction Primary production Phosphorous concentration Correlational study Manipulative study
University of Ottawa - BIO 5901 © Scott Findlay 01/05/2015 6:10 PM 11 Hypotheses and predictions, If the hypothesis could be true, yet the prediction not be observed, then … … the prediction is not derived deductively from the hypothesis, and thus it is not a prediction! … then the light will work if the old bulb is replaced (P) … then the light will work if the old bulb is replaced (P) If the light bulb is burnt out (H)… NB. True only if replacement bulb works! NB. True only if replacement bulb works!
University of Ottawa - BIO 5901 © Scott Findlay 01/05/2015 6:10 PM 12 Hypotheses, predictions or neither? “Chemical X will affect metamorphisis which is controlled by the thyroid axis” “There is a difference in root length between the GMO plant and the non GM plant due to the presence of herbicide.” “There is a difference between the species- area relationship for endemic and non- endemic species of ants in the West Indies.” “The number of species on an island determined by dispersal rate, which increases richness, and extinction rate, which decreases richness”
University of Ottawa - BIO 5901 © Scott Findlay 01/05/2015 6:10 PM 13 Hypothesis, prediction or neither? “ There is a relationship between breast cancer mortality rates and the use of atrazine (in weight per acre) in crop production” “The mean difference of plant species between the five habitats is not zero.” “Higher COSEWIC status designations for species at risk result from proportionally higher habitat losses and higher taxonomic appeal of the species to the public” “In fish oil supplements, the extent to which the compound biomagnifies is an important determinant of contamination levels”
University of Ottawa - BIO 5901 © Scott Findlay 01/05/2015 6:10 PM 14 The bottom line: hypotheses and predictions A (mechanistic causal) hypothesis specifies a mechanism. The associated predictions depend on the study – different studies, different predictions. A causal hypothesis can only make “directional” predictions. Is a prediction really a prediction? Acid test: could the hypothesis be true, and yet observations are inconsistent with the prediction? If so, then the prediction isn’t.
University of Ottawa - BIO 5901 © Scott Findlay 01/05/2015 6:10 PM 15 Biological predictions versus statistical null hypotheses If P is the pattern predicted by the hypothesis, then the null hypothesis is –P (“not P”) … … so that if one rejects the null [ -(-P)] =P, one has a pattern consistent with the hypothesis.
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