Controlled Experiments

Transcription

1 Suppose you were testing the effects of a drug. You believe that this medication will lower blood pressure. You have your experimental group take the drug and measure their blood pressure before taking the medication as well after and blood pressure decreased. Might there be other variables that caused the decrease? Ideally made up of an experimental and a control group that differ in only 1 factor, this being the factor that the experiment is designed to test. The control group remains constant during the experiment. Control group: Control groups are not manipulated. One way to pinpoint the effects specifically of the drug is to divide the group being tested into two groups. Scientists will try to control both groups to keep them as identical as possible and keep the environment as identical as possible. Did the blood pressure decrease because of the medication? They are the foundational point for which to compare the experimental group against. This group is identical to the control group EXCEPT that it is manipulated in ONE WAY. The blood pressure can be checked in both the group receiving the drug (experimental group) and the group that did not receive the drug (control group). The effects of the drug is now isolated since this is the ONLY thing that differed between the two groups. Test the effect of excessive water drinking has on humans. healthyweightlosshelp.info Experimental Group: Make a group of people drink a lot of water. What ever effects they display you might be inclined to attribute to excessive water drinking. Experimental group: The group being treated, or otherwise manipulated for the sake of the experiment. Then only one group will be given the drug. HOWEVER, how do you know that all the symptoms they display are from drinking lots of water?

2 Maybe some of the symptoms they display are totally unrelated to water and are the result of some other factor (like being together in an isolated group.) healthyweightlosshelp.info Therefore, in order to separate excessive water symptoms from other symptoms you gather a second group of people together and place them in exactly the same environment as the first group EXCEPT you do not give them excess water. This is your control group. Another example of a controlled experiment Hypothesis: Plants will grow taller when given Miracle Grow. Plants A & B are both given the same amount of light, water, and are stored at the same temperature. Plant A is given Miracle Grow. UK/Safety/prohibition What ever differences there are between your experimental group and your control group are due solely to excess water. Test: All the variables are kept constant except the one you are testing. Another example of a controlled experiment Plants A & B are both given the same amount of light, water, and are stored at the same temperature. Plant A is given Miracle Grow. Independent Variable - the factor you change, what you do to your exp. group. What is the Independent variable? Necessity of a control Controls are needed to eliminate alternate explanations of experimental results. Miracle Grow Dependent Variable - what happens as a result of that treatment, what you are measuring What is the dependent Variable? Height of the plant

3 Quantitative Data Types of Research & Graphing Your Data Data in tables and graphs may make patterns otherwise obvious Often, results of experiments only make sense after a lot of analysis of data Why do Warning Colorations exist in nature? # These bright colors and patterns mark animal It is useful to put quantitative data into as dangerous to potential predators. a table or graph A picture is worth a thousand words Ex: An experiment about mimicry This is an adaptation = it increases the fitness of an animal - the ability to survive and reproduce successfully CaseMimicry Study of Hypothesis-Based Science Case StudyAon One way to learn more about how hypothesis-based science works is to examine a case study. In contrast to a made-up example, such as the flashlight problem, a case s in law school train to become lawyers by analyzing the actual documents of past legal cases. Similarly, a case study of a research project that was published in a Eastern coral snake - poisonous with red, yellow Batesian Mimicry = Involves imposters that look e rest of this section is a case study of an inquiry about what harmless snakes might gain by imi and black rings like poisonous species but are harmless to Predators rarely attack this snake From Observations to Question and Hypothesis predators Scarlot king snake - mimics the ringed coloration The story begins with some key observations. Many poisonous animals are brightly colored, with distinctive patterns in some species. This appearance is called warn of the coral snake e also Ex: two wasps andimposters two insects mimics. These look like a poisonous species but are really harmless to predators. For example, a non-stinging insect called the flower fly is very si ions is: What is the function of such mimicry? that are not dangerous but look like the harmful wasp In 2001, a team of biologists designed a simple but clever set of experiments to test a hypothesis that was first suggested over a century earlier. Here's the hypothe al harmful species. Researchers David and Karin Pfennig, along with one of their college students, tested this hypothesis by studying mimicry in snakes that live in N ngs of red, yellow, and black. Predators rarely attack these snakes. A nonpoisonous snake named the scarlet king snake m Researchers David & Karin Pfenning Thought that maybe Mimics benefit because Testing a Prediction of the Hypothesis confuse them with actual harmful species What is predators the explanation for this case of look-alike snakes? According to the mimicry hypothesis, the coral-snakelike appearance of king snakes repels predators. Th To test this prediction, the researchers made hundreds, and black less frequently than they will attack snakes lacking such warning coloration. of artificial king sn ificial snakes: those with the red, yellow, and black ring pattern of coral snakes; and snakes with The researchers placed equal numbers of the two types of artificial snakes in various sites throughout North and South Carolina. After four weeks, the team retrieved

4 What is the explanation for this case of look-alike snakes? Hypothesis = The coral snake-like appearance of king snakes repels predators Prediction = Predators attack snakes with the bright rings of red, yellow, and black less frequently than they attack snakes lacking these warning coloration Researchers made artificial snakes with either red, yellow, black ring pattern or plain brown color. Put snakes in same environment in North Carolina and South Carolina as the live snakes After 4 weeks = counted bite / claw marks Predators include foxes, coyotes, raccoons, and black bears. Why did the experiment have both fake ringed and plain brown snakes? The contrast in coloration was needed to see if predators attack snakes based on their color. What is the variable being manipulated? Presence or absence of colored rings. This experiment tests the effects of a single variable = Controlled Experiment Scientists try in controlled experiments to eliminate (control) other variables that could affect outcome. Experiment - designed to test the effect of the colored rings What is the Experimental Group? Artificial snakes with colored rings What is the Control Group? Brown snakes - show what happens in the absence of colored rings

5 Everything else about the two snake groups was the same. the ringed snakes compared to the brown snakes could only have been due to the difference in Both placed at random in the same locations. coloration. of the predators varied, but both kinds of snakes were subject to the same variations. experiment only begin to analysis of the data. # For quantitative data, note again that it is often helpful to put the data in the form of a table or graph. Making pictures out of data The bar graph summarizes the results. A scientist carefully collects and organizes It reinforces the purpose of using two groups of snakes. 84 % of attacks were on the plain brown snakes 16 % for the snakes with colored rings. Data fit the prediction based on the mimicry hypothesis. The experiment supports the hypothesis that the king snakes' mimicry of coral-snake coloration helps protect against predators. Often, the results of an make sense after much Brown snakes controlled, or cancelled out, the effects of the unwanted variables, leaving colored rings as the only consistent difference between the two groups of snakes Now, any difference in the number of attacks on Both snakes were made of the same materials. # Conditions such as light, temperature, and appetite data from the experiment often in tables or graphs. How s a critter Graphs Examples: to choose? line graphs bar graphs (or histograms) graphing data that shows continuous change graphing data that is in disconnected groups

6 temperature How does elevation affect temperature? 5 Line graph elevation (feet above sea level) temp. ( C) ft? number of students Which drink do you like best? Bar graph drink number Coke 1 Pepsi 1 Water 4 Iced tea 4 Gatorade 3 Red Bull? elevation 0 Coke RB Pepsi RB water tea Gatorade type of drink Credibility in Science A hypothesis gains credibility by surviving repeated attempts to falsify it while testing eliminates (falsifies) alternative hypotheses. A THEORY: supported by a large body of evidence in comparison to a hypothesis broader in scope than a hypothesis more general than a hypothesis. More on Theories A theory is an explanation for natural events that is based on a large number of observations. Theories EXPLAIN what we observe. # For instance, the Germ Theory explains why we get sick and why we get infections $ It uses observation of viruses and bacteria as well as data from those who get ill to create a theory on what causes the illness.

7 More on Theories Theories are changeable and expandable, and most importantly, theories are FALSIFIABLE. In order to be a valid scientific theory there must be some way that an observation or experiment could prove it to be false if it is untrue. # For example, Einstein's theory of Relativity made predictions about the results of experiments. These experiments could have produced results that contradicted Einstein, so the theory was (and still is) falsifiable. So theories must explain a wide range of observations, be falsifiable, & can be changed if new evidence presents itself Are there Limitations to what Science can Study? Science requires natural explanations for natural phenomena. Observations and experimental results must be REPEATABLE (A sighting of the Virgin Mary cannot be repeated at will) Hypothesis must be FALSIFYABLE if those explanations are NOT true. (What experiment could ever be done to falsify the existence of unicorns?) Science cannot support or falsify supernatural explanations, which are outside the bounds of science. ANY QUESTIONS?