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by: Sally Jast


Sally Jast
GPA 3.64


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This 2 page Class Notes was uploaded by Sally Jast on Wednesday September 30, 2015. The Class Notes belongs to BIOL 283 at Western Kentucky University taught by Staff in Fall. Since its upload, it has received 33 views. For similar materials see /class/216721/biol-283-western-kentucky-university in Biology at Western Kentucky University.




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Date Created: 09/30/15
Biology 283 Lab Resources Hypothesis Testing Contemporary biological research is heavily dominated by the hypothesistesting approach which emphasizes asking and addressing discrete questions Many journals no longer publish purely descriptive studies though there will always be outlets for such data In hypothesistesting one relies on inferential statistics to make comparisons among variables or samples in the data set test predictions derived from working hypotheses etc The advantages of this approach are that 1 the study must be more thoroughly planned and considered from the outset making data collection more efficient and valid for the question at hand 2 data collection and analysis take a comparative approach often expanding the scope ofthe question beyond that seen in a descriptive study and 3 by using inferential tests of hypotheses one can generalize beyond the data at hand and make inferences about the populations from which the samples were drawn As such the conclusions of hypothesistesting science are generally more powerful and farreaching than those of descriptive science and are thus of interest to a greater number of scientists Hypothesistesting science is built squarely upon the foundations of the scientific method making observations developing a working hypothesis framing predictions and testing that hypothesis analyzing and interpreting data with respect to predictions repeating the process While this is true of any good scientific study even descriptive the key to the hypothesistesting approach is a cognizance of these steps in the design of the experiment or data collection strategy That is one must be wellaware ofwhere the research is headed priorto embarking on data collection and analysis In addition to maximizing the efficiency of data collection analyses based on such a priori hypotheses those generated prior to data collection generally have more statistical power and validity than those developed a posteriori post hoc after the fact A further distinction between descriptive and hypothesistesting studies has to do with the types of questions that one asks While descriptive studies address questions like 39What is the species composition of the flora of Lost River Cave39 a similar hypothesistesting study might ask 39Is there a statisticallysignificant difference between the flora of Lost River Cave and that of surrounding forest habitat39 or 39Do differences in temperature regimes or humidity gradients between Lost River Cave and surrounding forest habitats affect species composition and patterns of species abundance in these two areas 239 Notice in the latter examples the greater emphasis on comparison and collection of data with an explicit purpose in mind Key to hypothesistesting science is the ability to develop working hypotheses from a set of initial observations As a review working hypotheses are tentative causal explanations for a set of observations and must be logical and consistent with the data at hand However because no hypothesis can ever be proven to be true because to do so would require that it be true under all possible circumstances a conditions science proceeds by framing and testing null hypotheses Null hypotheses are typically stated as the negative of the working hypothesis for example that 39There is no difference in species composition or abundance between Lost River Cave and surrounding forest habitat If this null hypothesis is tested and shown to be statistically untrue this provides support for but not proof of the working hypothesis According to the philosopher of science Sir Karl Popper the key to science is falsifiability or the ability to disprove incorrect hypotheses Therefore science proceeds by formulating and attempting to disprove hypotheses particularly null hypotheses The fact that working hypotheses can at best be supported when null hypotheses are disproved would seem to be a rather weak way of advancing scientific knowledge leading to a 39hasn39t failed me yet39 criterion for success However the scientific method is actually much more powerful that that because wellphrased working hypotheses often lead to predictions that may themselves be tested Hypotheses that make no predictions are effectively worthless and hypotheses are evaluatedlacceptedldiscarded based on their predictive success Predictions of a working hypothesis lead to generation of alternative hypotheses specific hypotheses that may be supported ifthe null hypothesis is rejected These alternatives are derived from the working hypothesis but often provide more explicit mechanistic scenarios to account for the observations upon which the working hypothesis is based For example ifthe working hypothesis is that a difference in species compositionabundance exists between Lost River Cave and surrounding habitat one might generate alternatives such as 39Species more tolerant of cooler more shaded and humid and less variable conditions should be more abundant in Lost River Cave than in surrounding forest39 or 39The Lost River Cave valley should be generally cooler moister and more environmentally constant than surrounding forest leading to differences in species composition and abundance between the two areas39 So in reality from a single working hypothesis one often generates a null hypothesis to be tested by collection and analysis of additional data that of 39No difference39 as well as one or several alternative hypotheses that may be supported by the data ifthe null hypothesis is rejected Because one can generate multiple alternative hypotheses it is important to try and phrase alternatives so that they are mutually exclusive One additional concept to keep in mind is that acceptance or rejection of hypotheses is statistical not absolute When one concludes that there is a 39significant difference39 thus rejecting the null hypothesis it means only that there is a high probability that the null hypothesis is false that is it is highly unlikely that one would observe results as deviant from those expected by chance alone if the null hypothesis were true By convention we conclude that statistical significance exists that the null hypothesis is false ifthere is less than a fivepercent chance of getting the observed results if the null hypothesis were true the 005 criterion Nevertheless it is possible that a rejected null hypothesis is actually true and conversely that an accepted null hypothesis may in fact be false This uncertainty leads to the concept of Type and Type II errors which will be considered later In summary any experiment or data collectionanalysis program can be thought of as a simplified model of the real world Like any model the validity of the conclusions drawn is a function of how well the model represents reality and this is itself dependent on one39s prior understanding of the system If one constructs a mathematical or physical model of some aspect ofthe real world and that model leads to predictions that are supported by observations of nature we would conclude that the model is based on sound principles a good prior understanding of how variables interact and which are important to understanding the system Similarly if one can take a initial observations and frame a set of testable hypotheses of which some are then supported by data collection and analysis we would conclude that the interpretations are solid and based on a good understanding ofthe system under study As such it is essential that the entire research program be oriented towards addressing in the most efficient powerful way the question at hand In addition one should always try to frame questions so as to make them as farreaching as possible questions of broad general significance are always preferable to and often no more difficult to address than those which take a more narrow view of the world


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