Lab 01 Fall 2007 Experimental Design

Lab 01 Fall 2007 Experimental Design - EXPERIMENTAL DESIGN...

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EXPERIMENTAL DESIGN & SETUP Lab 1 1-1 Reminders! For this lab, you may get dirty, so dress appropriately. Key Concepts Experimental Design Hypothesis Testing Confounding Factors Bias Replication I. EXPERIMENTAL DESIGN & HYPOTHESIS TESTING Experimental design refers to the careful planning of an experiment, from beginning to end, before it is conducted. It is perhaps the most important aspect of a scientific study. The results of ecological experiments should advance our understanding of nature; therefore, experiments must be set up in such a way that we can have confidence in the results/answers we obtain. If studies are carried out with no regard for the experimental design, the results are likely to be invalid or inconclusive. (See Type I and II Errors) The Question There are a few things you must consider when designing an experiment. First, you must identify the question that you are trying to answer. Researchers spend a good deal of time developing interesting and important questions that can be addressed through scientific methods. Most ecological questions arise from observations of natural phenomena and/or the human imagination. To develop an ecological question, you must first make an observation of some pattern or event. For example, you may notice that mice are more abundant in one habitat than another. Second, you must transform your observations into a question: Why are mice more abundant in grasslands than in forests? This is a fundamental question in ecology – why do species seem to be found in some places but not others? The answer to this question could even be directly relevant to human health because mice are vectors of several human diseases. Once you have identified your question, you need to think about how you can answer it. This will involve identifying the various factors that you believe may be responsible for the pattern, and ideally, manipulating the various factors (variables) in an experiment. For example, after observing the mice in the forest and grassland habitats, you noticed that 1) mice in both habitats were eating primarily grass seeds, and 2) there were far more grass seeds in the grassland than in the forest. From these observations, you may get the idea that more mice are found in the grassland because more food is available in the grassland. This educated guess or prediction is called the hypothesis . Hypothesis Versus Null Hypothesis In this lab, we will focus on hypothesis testing , a scientific method developed extensively by Popper (1968). This is the most widely used scientific method (Figure 1). One develops explicit hypotheses about a natural pattern or phenomenon. Following Popper's methodology, one can never prove a general hypothesis, one can only find support for a hypothesis or reject a hypothesis. The reason we don’t prove a hypothesis is that there may be another explanation for what we observe that we haven’t yet considered. Therefore, we say that our results are consistent with or support our hypothesis. Its
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Lab 01 Fall 2007 Experimental Design - EXPERIMENTAL DESIGN...

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