Chapter 1 Introduction to the Scientific Method Can Science Cure the Common Cold?

Transcription

1 Chapter 1 Introduction to the Scientific Method Can Science Cure the Common Cold? 1.1 The Process of Science Science is a body of knowledge and the process used to obtain the knowledge As knowledge, science is simply an array of facts and explanations of those facts. The processes to obtain information in science is called the scientific method: Observing Proposing ideas Testing Discarding those ideas that fail The Nature of Hypotheses Hypothesis: proposed explanation Testable - must be able to make observations and measurements to evaluate the hypothesis Falsifiable - the observations or measurements must potentially be able to proven the hypothesis false The Nature of Hypotheses Where do hypotheses come from? Both logical and creative influences are used Figure 1.1 Scientific Theory Powerful, broad explanation of a large set of observations Can explain some aspect of nature, for example Rests on many hypotheses that have been tested Generates additional hypotheses The Logic of Hypothesis Tests Example: consuming vitamin C decreases the risk of catching a cold Inductive reasoning: combining a series of specific observations into a generalization Jamie took vitamin C and did not get a cold Jesse took vitamin C and did not get a cold Jordan took vitamin C and did not get a cold Conclude: People with names starting with J that took vitamin C did not get a cold To test, make a prediction using deductive reasoning. Can be constructed similarly to an if then statement IF people who took vitamin C did not get a cold

2 THEN the people who take vitamin C will not get a cold The process looks something like this: Figure 1.3 A hypothesis that fails our test is rejected and considered disproven. A hypothesis that passes is supported, but not proven. Why not? An alternative hypothesis might be the real explanation. A different explanation might be better. 1.2 Hypothesis Testing The most powerful way to test hypotheses: do experiments Experiments support the hypothesis that the common cold is caused by a virus. Figure 1.4 The Experimental Method Experiments are contrived situations. Variables: factors that can change in value under different conditions Independent variables can be manipulated by the scientist (dosage of vitamin C to subjects in an experiment) Dependent variables cannot be changed by the researcher (susceptibility to colds by subjects in an experiment) Controlled variables are kept the same for all subjects (try to make sure nobody starts the experiment with a cold) Controlled Experiments Controlled experiment: tests the effect of a single variable Control: a subject who is not exposed to the experimental treatment Differences can be attributed to the experimental treatment. Controls and controlled variables are not the same thing. Example: Echinacea tea experiment: Hypothesis: drinking Echinacea tea relieves cold symptoms Experimental group drinks Echinacea tea 5-6 times daily. Control group drinks sham Echinacea tea (placebo). Both groups rated the effectiveness of their treatment on relieving cold symptoms. In one experiment, people who received echinacea tea felt that it was 33% more effective at reducing symptoms. Figure 1.7

3 Minimizing Bias in Experimental Design If human subjects know whether they have received the real treatment or a placebo, they may be biased. Blind experiment: subjects don t know what kind of treatment they have received Double blinding: the person administering the treatments also doesn t know until after the experiment is over gold standard for experimentation Using Correlation to Test Hypotheses It is not always possible or ethical to experiment on humans. Using existing data, is there a correlation between variables? A correlation is simply a relationship between two variables Hypothesis: stress makes people more susceptible to catching a cold Is there a correlation between stress and the number of colds people have caught? Results of such a study: the number of colds increases as stress levels increase. Figure 1.10 Caution! Correlation does not imply causation. The correlation might be due to other reasons. A cause is something that actually leads to an effect; it s not always easy to see what causes something to happen! Using Correlation to Test Hypotheses Figure Understanding Statistics Overview: What Statistical Tests Can Tell Us We can extend the results from small samples to an entire population. Find out if there is a difference between two samples: Is it real or due to chance? The Problem of Sampling Error Sampling error: the effect of chance We can calculate the probability that a result is simply due to sampling error. Statistically significant: an observed difference is probably not due to sampling error Standard error: the statistical variability in the data used to calculate the mean (average). Much population variation = large standard error (more spread-out results/data) Small population variation = small standard error (less spread-out results/data)

4 Factors that Influence Statistical Significance Sample size The true difference between populations Bigger is better: more likely to detect differences The Problem of Sampling Error Figure 1.14 What Statistical Tests Cannot Tell Us If an experiment was designed and carried out properly Can evaluate the probability of sampling error, not observer error May not be of any biological significance 1.4 Evaluating Scientific Information Primary Sources Researchers can submit a paper about their results to a professional journal (primary source). Peer review: evaluation of submitted papers by other experts Secondary sources include those sources such as books, news reports, the internet, and advertisements, which may provide some of the information contained in a primary source Information from Anecdotes Anecdotal evidence is based on one person s experience, not on experimental data. Example: a testimonial from a celebrity Science in the News Secondary sources may be missing critical information or report the information incorrectly. Consider the source of media reports. Be careful with the internet since anyone can post information. Be very cautious about claims made in paid advertisements. Understanding Science from Secondary Sources Use your understanding of the process of science to evaluate science stories. News media generally highlight only those science stories that seem newsworthy. They are more likely to report a positive result than a negative one. 1.5 Is There a Cure for the Common Cold? No, but prevention methods are known. Wash your hands! No effect on cold susceptibility:

5 Vitamin C Exposure to cold temperatures Exercise No vaccine for the common cold