The Scientific Method Tutorial Steps in the Scientific Method The Scientific Method Flowchart The Scientific Method in Detail Step 1: Observations Step 2: The Hypothesis Step 3: Testing the Hypothesis Step 4: Data Analysis Step 5: Stating Conclusions The Scientific Method Steps in the Scientific Method There is a great deal of variation in the specific techniques scientists use explore the natural world. However, the following steps characterize the majority of scientific investigations: Step 1: Make observations Step 2: Propose a hypothesis to explain observations Step 3: Test the hypothesis with further observations or experiments Step 4: Analyze data Step 5: State conclusions about hypothesis based on data analysis Each of these steps is explained briefly below, and in more detail later in this section. Step 1: Make observations A scientific inquiry typically starts with observations. Often, simple observations will trigger a question in the researcher's mind. Example: A biologist frequently sees monarch caterpillars feeding on milkweed plants, but rarely sees them feeding on other types of plants. She wonders if it is because the caterpillars prefer milkweed over other food choices. Step 2: Propose a hypothesis The researcher develops a hypothesis (singular) or hypotheses (plural) to explain these observations. A hypothesis is a tentative explanation of a phenomenon or observation(s) that can be supported or falsified by further observations or experimentation. Example: The researcher hypothesizes that monarch caterpillars prefer to feed on milkweed compared to other common plants. (Notice how the hypothesis is a statement, not a question as in step 1.) Step 3: Test the hypothesis The researcher makes further observations and/or may design an experiment to test the hypothesis. An experiment is a controlled situation created by a researcher to test the validity of a hypothesis. Whether further observations or an experiment is used to test the hypothesis will depend on the nature of the question and the practicality of manipulating the factors involved. Example: The researcher sets up an experiment in the lab in which a number of monarch caterpillars are given a choice between milkweed and a number of other common plants to feed on. Step 4: Analyze data The researcher summarizes and analyzes the information, or data, generated by these further observations or experiments. Example: In her experiment, milkweed was chosen by caterpillars 9 times out of 10 over all other plant selections. Step 5: State conclusions The researcher interprets the results of experiments or observations and forms conclusions about the meaning of these results. These conclusions are generally expressed as probability statements about their hypothesis. Example: She concludes that when given a choice, 90 percent of monarch caterpillars prefer to feed on milkweed over other common plants. Often, the results of one scientific study will raise questions that may be addressed in subsequent research. For example, the above study might lead the researcher to wonder why monarchs seem to prefer to feed on milkweed, and she may plan additional experiments to explore this question. For example, perhaps the milkweed has higher nutritional value than other available plants. Return to top of page The Scientific Method Flowchart The steps in the scientific method are presented visually in the following flow chart. The question raised or the results obtained at each step directly determine how the next step will proceed. Following the flow of the arrows, pass the cursor over each blue box. An explanation and example of each step will appear. As you read the example given at each step, see if you can predict what the next step will be. onclick="javascript:popUp3('Popups/Popup416.html','height=200,width=600,resizable=yes,scrollbars=yes,men ubar=yes,toolbar=no,location=no')"> onclick="javascript:popUp3('Popups/Popup414.html','height=200,width=600,resizable=yes,scrollbars=yes,men ubar=yes,toolbar=no,location=no')"> onclick="javascript:popUp3('Popups/Popup415.html','height=200,width=600,resizable=yes,scrollbars=yes,men ubar=yes,toolbar=no,location=no')"> onclick="javascript:popUp3('Popups/Popup413.html','height=200,width=600,resizable=yes,scrollbars=yes,men ubar=yes,toolbar=no,location=no')"> onclick="javascript:popUp3('Popups/Popup4- 12.html','height=200,width=600,resizable=yes,scrollbars=yes,men ubar=yes,toolbar=no,location=no')"> onclick="javascript:popUp3('Popups/Popup411.html','height=200,width=600,resizable=yes,scrollbars=yes,men ubar=yes,toolbar=no,location=no')"> onclick="javascript:popUp3('Popups/Popup410.html','height=200,width=600,resizable=yes,scrollbars=yes,men ubar=yes,toolbar=no,location=no')"> onclick="javascript:popUp3('Popups/Popup49.html','height=200,width=600,resizable=yes,scrollbars=yes,menu bar=yes,toolbar=no,location=no')"> onclick="javascript:popUp3('Popups/Popup48.html','height=200,width=600,resizable=yes,scrollbars=yes,menu bar=yes,toolbar=no,location=no')"> onclick="javascript:popUp3('Popups/Popup47.html','height=200,width=600,resizable=yes,scrollbars=yes,menu bar=yes,toolbar=no,location=no')"> onclick="javascript:popUp3('Popups/Popup46.html','height=200,width=600,resizable=yes,scrollbars=yes,menu bar=yes,toolbar=no,location=no')"> onclick="javascript:popUp3('Popups/Popup45.html','height=200,width=600,resizable=yes,scrollbars=yes,menu bar=yes,toolbar=no,location=no')"> onclick="javascript:popUp3('Popups/Popup44.html','height=200,width=600,resizable=yes,scrollbars=yes,menu bar=yes,toolbar=no,location=no')"> onclick="javascript:popUp3('Popups/Popup43.html','height=200,width=600,resizable=yes,scrollbars=yes,menu bar=yes,toolbar=no,location=no')"> onclick="javascript:popUp3('Popups/Popup42.html','height=200,width=600,resizable=yes,scrollbars=yes,menu bar=yes,toolbar=no,location=no')"> onclick="javascript:popUp3('Popups/Popup41.html','height=200,width=600,resizable=yes,scrollbars=yes,menu bar=yes,toolbar=no,location=no')"> Activity: Apply the Scientific Method to Everyday Life Use the steps of the scientific method described above to solve a problem in real life. Suppose you come home one evening and flick the light switch only to find that the light doesn’t turn on. What is your hypothesis? How will you test that hypothesis? Based on the result of this test, what are your conclusions? Follow your instructor's directions for submitting your response. The above flowchart illustrates the logical sequence of conclusions and decisions in a typical scientific study. There are some important points to note about this process: 1. The steps are clearly linked. The steps in this process are clearly linked. The hypothesis, formed as a potential explanation for the initial observations, becomes the focus of the study. The hypothesis will determine what further observations are needed or what type of experiment should be done to test its validity. The conclusions of the experiment or further observations will either be in agreement with or will contradict the hypothesis. If the results are in agreement with the hypothesis, this does not prove that the hypothesis is true! In scientific terms, it "lends support" to the hypothesis, which will be tested again and again under a variety of circumstances before researchers accept it as a fairly reliable description of reality. 2. The same steps are not followed in all types of research. The steps described above present a generalized method followed in a many scientific investigations. These steps are not carved in stone. The question the researcher wishes to answer will influence the steps in the method and how they will be carried out. For example, astronomers do not perform many experiments as defined here. They tend to rely on observations to test theories. Biologists and chemists have the ability to change conditions in a test tube and then observe whether the outcome supports or invalidates their starting hypothesis, while astronomers are not able to change the path of Jupiter around the Sun and observe the outcome! 3. Collected observations may lead to the development of theories. When a large number of observations and/or experimental results have been compiled, and all are consistent with a generalized description of how some element of nature operates, this description is called a theory. Theories are much broader than hypotheses and are supported by a wide range of evidence. Theories are important scientific tools. They provide a context for interpretation of new observations and also suggest experiments to test their own validity. Theories are discussed in more detail in another section. Recommended Reading • "A Method of Enquiry" by George Kneller, in Science and Its Ways of Knowing. • "The So-called Scientific Method" by Henry H. Bauer, in Science and Its Ways of Knowing. Return to top of page The Scientific Method in Detail In the sections that follow, each step in the scientific method is described in more detail. Step 1: Observations Observations in Science An observation is some thing, event, or phenomenon that is noticed or observed. Observations are listed as the first step in the scientific method because they often provide a starting point, a source of questions a researcher may ask. For example, the observation that leaves change color in the fall may lead a researcher to ask why this is so, and to propose a hypothesis to explain this phenomena. In fact, observations also will provide the key to answering the research question. In science, observations form the foundation of all hypotheses, experiments, and theories. In an experiment, the researcher carefully plans what observations will be made and how they will be recorded. To be accepted, scientific conclusions and theories must be supported by all available observations. If new observations are made which seem to contradict an established theory, that theory will be re-examined and may be revised to explain the new facts. Observations are the nuts and bolts of science that researchers use to piece together a better understanding of nature. Observations in science are made in a way that can be precisely communicated to (and verified by) other researchers. In many types of studies (especially in chemistry, physics, and biology), quantitative observations are used. A quantitative observation is one that is expressed and recorded as a quantity, using some standard system of measurement. Quantities such as size, volume, weight, time, distance, or a host of others may be measured in scientific studies. Some observations that researchers need to make may be difficult or impossible to quantify. Take the example of color. Not all individuals perceive color in exactly the same way. Even apart from limiting conditions such as colorblindness, the way two people see and describe the color of a particular flower, for example, will not be the same. Color, as perceived by the human eye, is an example of a qualitative observation. Qualitative observations note qualities associated with subjects or samples that are not readily measured. Other examples of qualitative observations might be descriptions of mating behaviors, human facial expressions, or "yes/no" type of data, where some factor is present or absent. Though the qualities of an object may be more difficult to describe or measure than any quantities associated with it, every attempt is made to minimize the effects of the subjective perceptions of the researcher in the process. Some types of studies, such as those in the social and behavioral sciences (which deal with highly variable human subjects), may rely heavily on qualitative observations. Question: Why are observations important to science? Limits of Observations Because all observations rely to some degree on the senses (eyes, ears, or steady hand) of the researcher, complete objectivity is impossible. Our human perceptions are limited by the physical abilities of our sense organs and are interpreted according to our understanding of how the world works, which can be influenced by culture, experience, or education. According to science education specialist, George F. Kneller, "Surprising as it may seem, there is no fact that is not colored by our preconceptions" ("A Method of Enquiry," from Science and Its Ways of Knowing [Upper Saddle River: Prentice-Hall Inc., 1997], 15). Observations made by a scientist are also limited by the sensitivity of whatever equipment he is using. Research findings will be limited at times by the available technology. For example, Italian physicist and philosopher Galileo Galilei (1564–1642) was reportedly the first person to observe the heavens with a telescope. Imagine how it must have felt to him to see the heavens through this amazing new instrument! It opened a window to the stars and planets and allowed new observations undreamed of before. In the centuries since Galileo, increasingly more powerful telescopes have been devised that dwarf the power of that first device. In the past decade, we have marveled at images from deep space, courtesy of the Hubble Space Telescope, a large telescope that orbits Earth. Because of its view from outside the distorting effects of the atmosphere, the Hubble can look 50 times farther into space than the best earth-bound telescopes, and resolve details a tenth of the size (Seeds, Michael A., Horizons: Exploring the Universe, 5th ed. [Belmont: Wadsworth Publishing Company, 1998], 86-87). Construction is underway on a new radio telescope that scientists say will be able to detect electromagnetic waves from the very edges of the universe! This joint U.S.-Mexican project may allow us to ask questions about the origins of the universe and the beginnings of time that we could never have hoped to answer before. Completion of the new telescope is expected by the end of 2001. Although the amount of detail observed by Galileo and today's astronomers is vastly different, the stars and their relationships have not changed very much. Yet with each technological advance, the level of detail of observation has been increased, and with it, the power to answer more and more challenging questions with greater precision. Question: What are some of the differences between a casual observation and a 'scientific observation'? Return to top of page Step 2: The Hypothesis A hypothesis is a statement created by the researcher as a potential explanation for an observation or phenomena. The hypothesis converts the researcher's original question into a statement that can be used to make predictions about what should be observed if the hypothesis is true. For example, given the hypothesis, "exposure to ultraviolet (UV) radiation increases the risk of skin cancer," one would predict higher rates of skin cancer among people with greater UV exposure. These predictions could be tested by comparing skin cancer rates among individuals with varying amounts of UV exposure. Note how the hypothesis itself determines what experiments or further observations should be made to test its validity. Results of tests are then compared to predictions from the hypothesis, and conclusions are stated in terms of whether or not the data supports the hypothesis. So the hypothesis serves a guide to the full process of scientific inquiry. The Qualities of a Good Hypothesis • A hypothesis must be testable or provide predictions that are testable. It can potentially be shown to be false by further observations or experimentation. • A hypothesis should be specific. If it is too general it cannot be tested, or tests will have so many variables that the results will be complicated and difficult to interpret. A well-written hypothesis is so specific it actually determines how the experiment should be set up. • A hypothesis should not include any untested assumptions if they can be avoided. The hypothesis itself may be an assumption that is being tested, but it should be phrased in a way that does not include assumptions that are not tested in the experiment. • It is okay (and sometimes a good idea) to develop more than one hypothesis to explain a set of observations. Competing hypotheses can often be tested side-by-side in the same experiment. Question: Why is the hypothesis important to the scientific method? Activity: Recognizing Good Hypotheses Which of the following represents the best hypothesis? Click to select one, then check your answer. Cultures of the bacteria E. coli grow well in a lighted incubator maintained at 90°F. A culture of E. coli was accidentally left uncovered overnight on a laboratory bench where it was dark and temperatures fluctuated between 65°F and 68°F. When the technician returned in the morning, all the cells were dead. Which of the following statements is the best hypothesis to explain why the cells died, based on this observation? A. Some factor caused the E. coli cells to die. B. E. coli cells will die within 8 hours if it is exposed to temperatures below 80°F. C. E. coli cells were killed by the combined effects of lack of light, lack of moisture, exposure to oxygen, and lower than optimal temperatures. D. E. coli cells were killed intentionally by someone who entered the lab during the night, possibly a janitor. Return to top of page Step 3: Testing the Hypothesis A hypothesis may be tested in one of two ways: by making additional observations of a natural situation, or by setting up an experiment. In either case, the hypothesis is used to make predictions, and the observations or experimental data collected are examined to determine if they are consistent or inconsistent with those predictions. Hypothesis testing, especially through experimentation, is at the core of the scientific process. It is how scientists gain a better understanding of how things work. Testing a Hypothesis by Observation Some hypotheses may be tested through simple observation. For example, a researcher may formulate the hypothesis that the sun always rises in the east. What might an alternative hypothesis be? If his hypothesis is correct, he would predict that the sun will rise in the east tomorrow. He can easily test such a prediction by rising before dawn and going out to observe the sunrise. If the sun rises in the west, he will have disproved the hypothesis. He will have shown that it does not hold true in every situation. However, if he observes on that morning that the sun does in fact rise in the east, he has not proven the hypothesis. He has made a single observation that is consistent with, or supports, the hypothesis. As a scientist, to confidently state that the sun will always rise in the east, he will want to make many observations, under a variety of circumstances. Note that in this instance no manipulation of circumstance is required to test the hypothesis (i.e., you aren't altering the sun in any way). Testing a Hypothesis by Experimentation An experiment is a controlled series of observations designed to test a specific hypothesis. In an experiment, the researcher manipulates factors related to the hypothesis in such a way that the effect of these factors on the observations (data) can be readily measured and compared. Most experiments are an attempt to define a cause-and-effect relationship between two factors or events—to explain why something happens. For example, with the hypothesis "roses planted in sunny areas bloom earlier than those grown in shady areas," the experiment would be testing a causeand-effect relationship between sunlight and time of blooming. A major advantage of setting up an experiment versus making observations of what is already available is that it allows the researcher to control all the factors or events related to the hypothesis, so that the true cause of an event can be more easily isolated. In all cases, the hypothesis itself will determine the way the experiment will be set up. For example, suppose my hypothesis is "the weight of an object is proportional to the amount of time it takes to fall a certain distance." How would you test this hypothesis? The Qualities of a Good Experiment Experiments can vary considerably depending upon the hypothesis that is being tested. However, most experiments have the following elements in common. • The experiment must be conducted on a group of subjects that are narrowly defined and have certain aspects in common. This is the group to which any conclusions must later be confined. (Examples of possible subjects: female cancer patients over age 40, E. coli bacteria, red giant stars, the nicotine molecule and its derivatives.) • All subjects of the experiment should be (ideally) completely alike in all ways except for the factor or factors that are being tested. Factors that are compared in scientific experiments are called variables. A variable is some aspect of a subject or event that may differ over time or from one group of subjects to another. For example, if a biologist wanted to test the effect of nitrogen on grass growth, he would apply different amounts of nitrogen fertilizer to several plots of grass. The grass in each of the plots should be as alike as possible so that any difference in growth could be attributed to the effect of the nitrogen. For example, all the grass should be of the same species, planted at the same time and at the same density, receive the same amount of water and sunlight, and so on. The variable in this case would be the amount of nitrogen applied to the plants. The researcher would not compare differing amounts of nitrogen across different grass species to determine the effect of nitrogen on grass growth. What is the problem with using different species of plants to compare the effect of nitrogen on plant growth? • • There are different kinds of variables in an experiment. A factor that the experimenter controls, and changes intentionally to determine if it has an effect, is called an independent variable. A factor that is recorded as data in the experiment, and which is compared across different groups of subjects, is called a dependent variable. In many cases, the value of the dependent variable will be influenced by the value of an independent variable. The goal of the experiment is to determine a cause-and-effect relationship between independent and dependent variables—in this case, an effect of nitrogen on plant growth. In the nitrogen/grass experiment, (1) which factor was the independent variable? (2) Which factor was the dependent variable? • • Nearly all types of experiments require a control group and an experimental group. The control group generally is not changed in any way, but remains in a "natural state," while the experimental group is modified in some way to examine the effect of the variable which of interest to the researcher. The control group provides a standard of comparison for the experimental groups. For example, in new drug trials, some patients are given a placebo while others are given doses of the drug being tested. The placebo serves as a control by showing the effect of no drug treatment on the patients. In research terminology, the experimental groups are often referred to as treatments, since each group is treated differently. In the experimental test of the effect of nitrogen on grass growth, what is the control group? • • In the example of the nitrogen experiment, what is the purpose of a control group? • In research studies a great deal of emphasis is placed on repetition. It is essential that an experiment or study include enough subjects or enough observations for the researcher to make valid conclusions. The two main reasons why repetition is important in scientific studies are (1) variation among subjects or samples and (2) measurement error. Variation among Subjects There is a great deal of variation in nature. In a group of experimental subjects, much of this variation may have little to do with the variables being studied, but could still affect the outcome of the experiment in unpredicted ways. For example, in an experiment designed to test the effects of alcohol dose levels on reflex time in 18- to 22-year-old males, there would be significant variation among individual responses to various doses of alcohol. Some of this variation might be due to differences in genetic makeup, to varying levels of previous alcohol use, or any number of factors unknown to the researcher. Because what the researcher wants to discover is average dose level effects for this group, he must run the test on a number of different subjects. Suppose he performed the test on only 10 individuals. Do you think the average response calculated would be the same as the average response of all 18- to 22-year-old males? What if he tests 100 individuals, or 1,000? Do you think the average he comes up with would be the same in each case? Chances are it would not be. So which average would you predict would be most representative of all 18- to 22-year-old males? A basic rule of statistics is, the more observations you make, the closer the average of those observations will be to the average for the whole population you are interested in. This is because factors that vary among a population tend to occur most commonly in the middle range, and least commonly at the two extremes. Take human height for example. Although you may find a man who is 7 feet tall, or one who is 4 feet tall, most men will fall somewhere between 5 and 6 feet in height. The more men we measure to determine average male height, the less effect those uncommon extreme (tall or short) individuals will tend to impact the average. Thus, one reason why repetition is so important in experiments is that it helps to assure that the conclusions made will be valid not only for the individuals tested, but also for the greater population those individuals represent. "The use of a sample (or subset) of a population, an event, or some other aspect of nature for an experimental group that is not large enough to be representative of the whole" is called sampling error (Starr, Cecie, Biology: Concepts and Applications, 4th ed. [Pacific Cove: Brooks/Cole, 2000], glossary). If too few samples or subjects are used in an experiment, the researcher may draw incorrect conclusions about the population those samples or subjects represent. Use the jellybean activity below to see a simple demonstration of samping error. Directions: There are 400 jellybeans in the jar. If you could not see the jar and you initially chose 1 green jellybean from the jar, you might assume the jar only contains green jelly beans. The jar actually contains both green and black jellybeans. Use the "pick 1, 5, or 10" buttons to create your samples. For example, use the "pick" buttons now to create samples of 2, 13, and 27 jellybeans. After you take each sample, try to predict the ratio of green to black jellybeans in the jar. How does your prediction of the ratio of green to black jellybeans change as your sample changes? Conclusion Measurement Error The second reason why repetition is necessary in research studies has to do with measurement error. Measurement error may be the fault of the researcher, a slight difference in measuring techniques among one or more technicians, or the result of limitations or glitches in measuring equipment. Even the most careful researcher or the best state-of-the-art equipment will make some mistakes in measuring or recording data. Another way of looking at this is to say that, in any study, some measurements will be more accurate than others will. If the researcher is conscientious and the equipment is good, the majority of measurements will be highly accurate, some will be somewhat inaccurate, and a few may be considerably inaccurate. In this case, the same reasoning used above also applies here: the more measurements taken, the less effect a few inaccurate measurements will have on the overall average. Return to top of page Step 4: Data Analysis In any experiment, observations are made, and often, measurements are taken. Measurements and observations recorded in an experiment are referred to as data. The data collected must relate to the hypothesis being tested. Any differences between experimental and control groups must be expressed in some way (often quantitatively) so that the groups may be compared. Graphs and charts are often used to visualize the data and to identify patterns and relationships among the variables. Statistics is the branch of mathematics that deals with interpretation of data. Data analysis refers to statistical methods of determining whether any differences between the control group and experimental groups are too great to be attributed to chance alone. Although a discussion of statistical methods is beyond the scope of this tutorial, the data analysis step is crucial because it provides a somewhat standardized means for interpreting data. The statistical methods of data analysis used, and the results of those analyses, are always included in the publication of scientific research. This convention limits the subjective aspects of data interpretation and allows scientists to scrutinize the working methods of their peers. Why is data analysis an important step in the scientific method? Return to top of page Step 5: Stating Conclusions The conclusions made in a scientific experiment are particularly important. Often, the conclusion is the only part of a study that gets communicated to the general public. As such, it must be a statement of reality, based upon the results of the experiment. To assure that this is the case, the conclusions made in an experiment must (1) relate back to the hypothesis being tested, (2) be limited to the population under study, and (3) be stated as probabilities. The hypothesis that is being tested will be compared to the data collected in the experiment. If the experimental results contradict the hypothesis, it is rejected and further testing of that hypothesis under those conditions is not necessary. However, if the hypothesis is not shown to be wrong, that does not conclusively prove that it is right! In scientific terms, the hypothesis is said to be "supported by the data." Further testing will be done to see if the hypothesis is supported under a number of trials and under different conditions. If the hypothesis holds up to extensive testing then the temptation is to claim that it is correct. However, keep in mind that the number of experiments and observations made will only represent a subset of all the situations in which the hypothesis may potentially be tested. In other words, experimental data will only show part of the picture. There is always the possibility that a further experiment may show the hypothesis to be wrong in some situations. Also, note that the limits of current knowledge and available technologies may prevent a researcher from devising an experiment that would disprove a particular hypothesis. The researcher must be sure to limit his or her conclusions to apply only to the subjects tested in the study. If a particular species of fish is shown to consume their young 90 percent of the time when raised in captivity, that doesn't necessarily mean that all fish will do so, or that this fish's behavior would be the same in its native habitat. Finally, the conclusions of the experiment are generally stated as probabilities. A careful scientist would never say, "drug x kills cancer cells;" she would more likely say, "drug x was shown to destroy 85 percent of cancerous skin cells in rats in lab trials." Notice how very different these two statements are. There is a tendency in the media and in the general public to gravitate toward the first statement. This makes a terrific headline and is also easy to interpret; it is absolute. Remember though, in science conclusions must be confined to the population under study; broad generalizations should be avoided. The second statement is sound science. There is data to back it up. Later studies may reveal a more universal effect of the drug on cancerous cells, or they may not. Most researchers would be unwilling to stake their reputations on the first statement. As a student, you should read and interpret popular press articles about research studies very carefully. From the text, can you determine how the experiment was set up and what variables were measured? Are the observations and data collected appropriate to the hypothesis being tested? Are the conclusions supported by the data? Are the conclusions worded in a scientific context (as probability statements) or are they generalized for dramatic effect? In any researched-based assignment, it is a good idea to refer to the original publication of a study (usually found in professional journals) and to interpret the facts for yourself. Activity: Interpretation of a Science Study as Presented in the Popular Media Read the article below, and then answer the questions that follow. Follow your instructor's directions for submitting your responses. E. Coli Kills Cancer Cancer is often fought with chemotherapy, and the effects of these toxic drugs can be excruciating. But Canadian researchers have discovered that a familiar, yet potent toxin can actually shrink brain tumors in less than 48 hours with no apparent ill effects. The cancer-fighting chemical is verotoxin, which is produced by the ubiquitous E.coli bacteria. This toxin, which causes diarrhea, was injected into human brain tumors implanted in mice. It not only shrank the tumors, but none of the tumors reappeared. How can a substance dangerous in the stomach not be dangerous to the brain cells? "What is important is the amount of the toxin," says Dr. Clifford Lingwood of the Hospital for Sick Children in Toronto. Just a little bit of it won't hurt you, but the more you're exposed to the sicker you'll get. The idea is to find a level that is harmless to the animal as a whole, but deadly to the cancer cells. A study of baboons measured how much verotoxin it would take to make an ape sick. Animals given small doses showed no side effects, nor did the mice in Lingwood's study. Lingwood says that the verotoxin stops the growth of new blood vessels. "Tumor cells are particularly susceptible," he explains, because the tumors are marked by a specific glycolipid, a receptor that acts as a gateway into the cell. The verotoxin finds the glycolipids on tumors and the blood vessels that surround the tumor cells. It attaches itself to the receptor and causes the cells to commit suicide. Verotoxins ignore normal, noncancerous brain cells, which don't contain the receptor. With the toxin attacking both its outer membrane and its food supply, the brain tumor shrivels almost immediately after treatment begins. In cancer cells in Petri dishes, "You can see a significant difference in 90 minutes," says Lingwood. Lingwood's results were reported in the June issue of the journal, Oncology Research. —Martha Heil Posted 7/19/1999 Answer the following questions: 1 What experiments did the scientists perform? 2 What hypothesis was tested? 3 What data were collected? 4 What were the conclusions of the study? 5 Was the data collected sufficient to support the conclusions made? 6 Was any important information about how the study was performed left out of the article? If so, what information is missing? 7 Is the title of this article an accurate statement of the study’s findings? Why or why not? Return to top of page Qualities of a Good Experiment The following are qualities of a good experiment: • narrowly defined subjects • all subjects treated alike except for the factor or variable being studied • a control group is used for comparison • measurements related to the factors being studied are carefully recorded • enough samples or subjects are used so that conclusions are valid for the population of interest • conclusions made relate back to the hypothesis, are limited to the population being studied, and are stated in terms of probabilities Recommended Reading • For more detailed information on experiments and different types of experimental designs see Chapter 2, "Establishing Causal Links," of A Beginner's Guide to the Scientific Method by Stephen S. Carey. Return to top of page
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