Designing a Controlled Experiment

Transcription

1 Designing a Controlled Experiment -The independent variable is real vs. artificial trees. The dependent variable is vine growth. -Do vines grow better on real trees? -If vines grow better on real trees, and we grow vines on artificial trees, then they will grow less. If vines do not depend on trees, and we grow vines on artificlal trees, then there will be no difference in growth. -The best way to analyze this data would be to average the measurements from each day for each group and compare the two groups on a x/y scatter or line graph. -From this data we can infer that vines grow better on real trees. -If we assume all variables were properly controlled, this experiment, data, analysis and conclusions are valid. -WHY? HOW? Design experiments to block the vines from getting nutrients from the tree to determine how. 6

2 1-2 How Scientists Work Redi s Test of Spontaneous Generation Independent Variable Materials Screen No Screen Dependent Variable (maggots) Experimental Control -Spontaneous generation is the sudden formation of life from non-living matter. -Through observation, Redi inferred that maggots hatch from eggs laid by flies. -If maggots hatch from fly eggs, and we allow meat to age in a covered jar, then maggots will not spontaneously appear. If maggots do not hatch from fly eggs (spontaneously generate), and we allow meat to age in a covered jar, then maggots will appear just as in an open jar. -Maggots come from eggs laid by flies and do not spontaneously generate from meat! -Redi s experiment is valid if we assume he controlled all other variables. 7

3 Conflicting Results & Critical Thinking Needham vs. Spallanzani Repeating Investigations & Critical Thinking Flask with gravy Gravy is boiled. Gravy is boiled. -Needham s results and conclusions contradict Redi s concerning spontaneous generation. -Needham s experiment is not valid because he had no control group and simply heating does not kill microorganisms. -Spallanzani s experiment is an improvement on Needham s and is valid. -Spallanzani s results contradict Needham s and agree with Redi s, refuting spontaneous generation. -This series of experiments did not once and for all convince the entire scientific community that life does not spontaneously generate. Some believed there was a vital force in the air that was expelled, or destroyed by boiling. Flask is open. Gravy is teeming with microorganisms. Flask is sealed. Gravy is free of microorganisms. 8

4 Adjusting the Experimental Design Louis Pasteur's Test of Spontaneous Generation Dependent Variable Materials Microorganisms No microorganisms until the curved-neck is broken. No Boil Boil Independent Variable Control Experimental -Pasteur s experiment is designed differently than Spallanzani s because he used curved-neck flasks that are not completely sealed so they are open to any vital force that may be in the air. -If there is no vital force allowing for spontaneous generation, and we boil broth in an unsealed curved-neck flask, then microorganisms will not spontaneously appear in the broth. -If microorganisms come from particles in the air, and we boil broth in an unsealed curved-neck flask, then microorganisms will not spontaneously appear in the broth. -Pasteur s experiment is valid if we assume he controlled all the variables! -Pasteur s results contradict Needham s, but agree with Spallanzani s and Redi s... NO SPONTANEOUS 9 GENERATION!!!

5 When Experiments Are Not Possible (alternative investigation) Ethical Concerns In Animal Testing - A non-experimental investigation is one where the answer to a question can not be found through scientific experimentation. -For example, questions concerning animal behavior, such as Where do Green Sea turtles go at night? can be best answered through observation. -Ethical concerns involving the use of living experimental subjects/models arise when the organism may experience discomfort, pain or death as a result of experimental treatment. -For ethical concerns, humans often can not be used as experimental subjects, therefore a closely related stand-in species, such as mice, may be used to model results comparable to humans. -One of the most widely used model organisms are mice because they share 85% of their genome with humans, are easy to maintain in the laboratory and reproduce quickly. 10