Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Chapter 2 Scientific Methods in Psychology Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Scientific Methods in Psychology Science is a word derived from Latin roots: “scientia” meaning “knowledge.” Scientific practice helps psychologists to know that they have obtained the most accurate and useful knowledge of mental processes and human behavior. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology The Scientific Method Why do we need it? The scientific method provides guidelines for scientists in all fields, including psychology, to use in evaluating discrete claims (called hypotheses) and broader theories. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology The Scientific Method Why do we need it? It is almost impossible to prove with utter certainty that any individual claim or theory is true beyond a doubt. The scientific method allows us to declare our conclusions to be probable to the point where it is reasonable to treat them as factual. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Scientific Theories What is a theory? A theory is a comprehensive explanation of observable events and conditions. A good theory makes precise and consistent predictions while relying on a small number of underlying assumptions. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Scientific Theories The importance of falsifiability A theory that makes precise predictions is falsifiable because it is easy to think of evidence that would confirm or contradict the theory. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Scientific Theories Example of a falsifiable scientific theory: Gravity is a force that pulls objects in the universe towards each other. According to the theory of gravity, larger and more massive objects pull smaller objects towards them. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology The Scientific Method How do we support claims scientifically? Scientists want to know the evidence that will support or disprove a claim. The scientific word for a claim is hypothesis. A hypothesis is a testable prediction of what will occur under a stated set of conditions. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Figure 2.1 A hypothesis leads to predictions. An experimental method tests those predictions; a confirmation of a prediction supports the hypothesis; a disconfirmation indicates a need to revise or discard the hypothesis. Conclusions remain tentative, especially after only one experiment. Most scientists avoid saying that their results “prove” a conclusion. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology The Scientific Method What’s the hypothesis? Claim: There is a relationship between televised violence and aggressive behavior. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology The Scientific Method How do we test the hypothesis? Here are some possible strategies: Measure how much time a sample of children watch violent television programs and compare that to how much violent behavior the children exhibit (This is an ex correlational study b/c you are comparing two things.) Have a group of children watch violent programs and another group watch non-violent programs, and then record the differences in amount of violent behavior between the two groups (an experimental study.) Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology The Scientific Method How do we measure the results? It is tricky to measure phenomena such as “violent behavior.” We need to operationally define concepts such as this one; clearly stating which behaviors will represent the phenomenon of interest (verbal threats, hitting, etc.). Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology The Scientific Method What do our results mean? If the results support the original prediction, it may mean the hypothesis is valid, but that does not eliminate other possible explanations for the outcome. If the results contradict the original prediction, the hypothesis may need to be modified or abandoned (at least under certain circumstances.) Scientists generally do not make any dramatic alterations to their conclusions based on one study only. They usually do longitudinal studies or more than one study and compare data. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology The Scientific Method Can you think of examples of evidence that would confirm or contradict this theory? EASILY. It is falsifiable. Is this claim falsifiable? “You will encounter new challenges in your travels this week.” NO IT IS to vague Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Psychology as a Science Science does not deal with proof or certainty. The history of science is one of constant revision in the face of new and compelling evidence. Yet in psychology and all other sciences, we apply the rigorous and systematic methods of scientific study – hypothesis, methods, results, and interpretation - to ensure that our claims are firmly grounded and our revisions reflect an improved understanding of the phenomena under scrutiny. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Definitions of Psychological Terms The problems of measurement We need to measure the phenomena we are studying. Sometimes what we study in psychology is not tangible. It is not as we are measuring weight or length of time. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Definitions of Psychological Terms The problems of Measurements In order to accurately measure these concepts and phenomena, we develop behavioral or observable definitions of them. We call these definitions operational definitions. An operational definition is one that specifies the operations or procedures used to produce or measure something. It’s a way to give an intangible idea a numerical value. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Definitions of Psychological Terms So if we are investigating the effect of watching violence on television on children’s aggressive behavior: We need to operationalize “violence” on television. We need to operationalize “aggressive behavior.” Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Definitions of Psychological Terms Violence might be operationalized as “the number of times in a one-hour show that one person threatens or injures another person.” Aggressive behavior might be operationalized as “the number of insults, threats and assaults by the subject over a 24-hour period after watching a particular television program”. (There are other versions of both of these operational definitions that would work well.) Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Concept Check Operational definitions Which of the following might be used as an operational definition of “attraction”? A feeling of affection when two people are together. (1) The number of minutes during which two people are touching each other over a four-hour period. (2) (2) Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Concept Check Which of the following might be used as an operational definition of “assertiveness?” The number of times a person makes requests or states his or her feelings over the course of a one-hour interaction. (1) An appearance of confidence and ease in social situations. (2) (1) Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Population Samples Usually in research we are asking questions that are pertinent to a large population of interest such as: Seven to ten-year-old children People diagnosed with depression Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Population Samples But it is not practical to study all the individuals in the population. We take a relatively small number of observations or individuals from the population, and we generalize from that small number. The small number of individuals or observations is called a sample. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Population Samples There are several types of samples and sampling procedures: A convenience sample is a group chosen because of its ease of availability and study. A representative sample closely resembles the population in its percentage of males and females, ethnic or racial groups, age levels, or whatever other characteristics might have some relevance to the results. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Population Samples A random sample is one in which every individual in the population has an equal chance of being selected. A cross-cultural sample is one that contains groups of people from at least two distinct cultures. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Population Samples How we go about obtaining a sample has to be carefully assessed in terms of our resources and goals. Sometimes it is acceptable and appropriate to rely on a convenience sample. Other times this strategy will produce results that are useless in helping us understand and interpret the “real world.” Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Concept Check Suppose I am interested in the attitudes of college students towards using the Internet in their studies. I survey my students in one Introductory Psychology class at my college. Can I assume that their attitudes are representative of the attitudes of all college students in general? Not a safe assumption – why? Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Experimenter Bias Because (fallible) humans do the research, we need to keep in check the various tendencies that can work to create erroneous research findings or erroneous interpretations of findings. Experimenter bias is the tendency of an experimenter to unintentionally distort the procedures or results of an experiment based on the expected or desired outcome of the research. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Experimenter Bias For example, if you were a researcher testing the hypothesis that children who have been diagnosed with learning disabilities are on average more creative than children who have no diagnosis, you may find it hard to “ignore your hypothesis” as you observe the children with an LD diagnosis going about whatever tasks you have devised to operationalize “creativity.” Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Experimenter Bias Methods have been devised to help counteract these normal human tendencies that create bias: Using blind observers who record data without knowing what the researcher is studying. Using a placebo control. A placebo is a pill or other sham treatment that makes it very difficult for the subjects (single-blind) or the subjects and experimenter (double-blind) to know who has received the treatment and who has not. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Table 2.2 Single-blind and double-blind Studies Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Research Design There are many methods used to study psychological concepts and phenomena. We start by asking ourselves “What happens, and under what circumstances does it seem to occur?” We try to choose the best procedure. Each method has advantages and disadvantages. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Observational (non-experimental) Research Design Naturalistic Observation: Careful monitoring and examination of what people and animals do under more or less natural circumstances. Example: Dr. Jane Goodall’s decades-long observation of chimpanzees in the forest of Gombe, recording their social organization and biological functioning. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Observational Research Design Case History: A thorough observation and description of a single individual, appropriate only when done for an unusual condition or circumstance. Example: The case of Phineas Gage, whose bizarre and unfortunate accident taught medical doctors and psychologists much about the nature of the prefrontal cortex of the brain. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Observational Research Design Survey: A survey is a study of the prevalence of certain beliefs, attitudes, or behaviors, based on people’s responses to specific questions. Example: Albert Kinsey’s 1948 survey of the sexual preferences and habits of Americans was ground breaking, although not by any means beyond criticism. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Observational Research Design Surveys A Few Concerns About Survey Research: Problems with obtaining a random or representative sample Competence or honesty of those who respond The wording of the questions Surveyor bias Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Correlational Studies Correlation Correlation is a measure of the relationship between two variables which are both outside of the investigator’s control. Examples of variables include aspects such as: height, weight, socio-economic level, number of years of education. The mathematical estimate of the strength and direction of a correlation is the correlation coefficient. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Correlational Studies The value of the correlation coefficient can range from – 1.00 to +1.00. The higher the absolute value, the stronger the relationship is, regardless of the direction. A negative correlation (-) means that as one variable increases, the other decreases. An example of a negative correlation is: The more absences a student has, the lower his or her grade in psychology is (more absences accompanied by fewer points on tests.) Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Figure 2.8abc In a scatterplot each dot represents data for one person; for example, each point in the center graph tells us one person’s weight and that person’s grade on the psychology final exam, in this case using hypothetical data. A positive correlation indicates that, as one variable increases, the other generally does also. A negative correlation indicates that, as one variable increases, the other generally decreases. The closer a correlation coefficient is to +1 or -1, the stronger the relationship. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Figure 2.8a In a scatterplot each dot represents data for one person; for example, each point in the center graph tells us one person’s weight and that person’s grade on the psychology final exam, in this case using hypothetical data. A positive correlation indicates that, as one variable increases, the other generally does also. A negative correlation indicates that, as one variable increases, the other generally decreases. The closer a correlation coefficient is to +1 or -1, the stronger the relationship. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Figure 2.8b In a scatterplot each dot represents data for one person; for example, each point in the center graph tells us one person’s weight and that person’s grade on the psychology final exam, in this case using hypothetical data. A positive correlation indicates that, as one variable increases, the other generally does also. A negative correlation indicates that, as one variable increases, the other generally decreases. The closer a correlation coefficient is to +1 or -1, the stronger the relationship. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Figure 2.8c In a scatterplot each dot represents data for one person; for example, each point in the center graph tells us one person’s weight and that person’s grade on the psychology final exam, in this case using hypothetical data. A positive correlation indicates that, as one variable increases, the other generally does also. A negative correlation indicates that, as one variable increases, the other generally decreases. The closer a correlation coefficient is to +1 or -1, the stronger the relationship. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Correlational Studies A positive correlation (+) means that as one variable increases, so does the other. An example of a positive correlation would be the higher the annual income, the greater the amount and number of donations to charity (more income accompanied by more charitable giving.) A zero or near zero correlation means that the variables have no relationship; that changes in one are not related to any type of change in the other. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Concept Check What type of correlation? People’s shoe size and IQ score Zero Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Concept Check The greater the number of years of education, the higher the income Positive Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Concept Check The greater the score on a depression inventory, the lower the score on a memory test Negative Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Concept Check Which relationship is stronger? +. 30 or -.90 -.90 Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Correlational Studies Some Problems with Interpreting Correlational Research: Illusory Correlation: An apparent relationship based on casual observations of unrelated or weakly related events. Example: The belief in “moon madness.” Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Correlational Studies Some Problems with Interpreting Correlational Research: Correlation Causation: Correlational research only tells us if two variables are related and how strongly. It does not tell us why – two conditions can appear together and yet not cause each other. Example: “The more someone weighs, the larger his or her vocabulary is.” Do you know why? Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Figure 2.9 A strong correlation between depression and impaired sleep does not tell us whether depression interferes with sleep, poor sleep leads to depression, or whether another problem leads to both depression and sleep problems. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Concept Check Interpreting correlational research Suppose we did a research study on our campus and found a -.75 correlation between frequency of exercise and level of depression. List all the possible conclusions that we might draw from this study. Exercising makes depression less likely. Depression makes exercising less likely. A third variable causes increases in exercise and decreases in depression. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Experiments Experiment: A study in which the investigator manipulates at least one variable (independent) while measuring at least one other variable (dependent). Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Figure 2.11 An experimenter manipulates the independent variable (in this case the programs people watch) so that two or more groups experience different treatments. Then the experimenter measures the dependent variable (in this case pulse rate) to see how the independent variable affected it. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Experiments Example: To test whether the hormone adrenaline enhances memory in mammals, a researcher teaches rats to run a maze. She gives a randomly selected portion of the rats a drug to block production of adrenaline. She then times all the rats on the maze. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Experiments Remember: In order for a study to be a true EXPERIMENT, one of the variables must be directly under the researcher’s control, and the other must be measurable in some scientific way. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Figure 2.12 Once researchers decide on the hypothesis they want to test, they must design the experiment. These procedures test the effects of watching televised violence. An appropriate, accurate method of measurement is essential. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Concept Check Is it an experiment? If so, name the independent and dependent variables. A researcher wants to know if men or women are better at a particular set of spatial relationship tasks. He compares a randomly selected group of 50 men and 50 women on a test of the task. Not an experiment – M/F is a subject variable, not a true independent variable. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Concept Check A researcher wants to know if a particular herbal supplement is helpful for improving memory. She selects 100 college sophomores who achieved an average score on a memory test, gives half of them the herb for one month, half of them an inert pill, and the re-tests them all. Yes IV = herb/no herb DV = score on memory test Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Experiments Other important terminology: Experimental group: The set of individuals who receive the treatment that the experiment is designed to test. Control group: The set of individuals who are treated in the same way as the experimental group except for the procedure that the experiment is designed to test. Random assignment: A selection method in which the experimenter assigns subjects to either the experimental or control group using a procedure based on chance. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Experiments Possible problems in carrying out and interpreting the results of experiments: Demand Characteristics: Cues that tell a subject what is expected of him or her, and what the researcher hopes to find. Example: If the subject knows that the drug being tested is supposed to improve mood, he or she may “feel better.” Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Experiments Possible problems in carrying out and interpreting the results of experiments: Ethical Considerations: In doing research with humans or animals, researchers must weigh possible harm that may be inflicted against the usefulness and other benefits that may be gained. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Ethical Concerns in Research Involving Human Subjects Safeguarding human subjects’ well-being: Use of informed consent: Subjects are advised on what to expect and explicitly state that they agree to continue. Institutional Research Board (IRB) Approval: A university or other reputable institution appoints a panel of qualified judges who review all research proposals before the actual study begins. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Ethical Concerns in Research Involving Human Subjects American Psychological Association standards: The criteria for appropriate treatment of humans who are experimental subjects are well known to members of this largest professional organization in the science. Censure and expulsion are possible consequences for those who do not follow these procedures. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Ethical Concerns in Research Involving Animals Though highly controversial, research studies that use animals to help us understand the body and brain have been essential to progress in medicine and psychology. Criteria for care and use of animals are established by professional organizations: APA The Neuroscience Society Animal care committees at research institutions Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology General Principles of Research Ethical Concerns in Research Involving Animals Following the guidelines, animal care committees strive to: Ensure that research animals are treated humanely Ensure that any pain and discomfort are kept to a minimum Ensure that all alternatives are examined before animals are subjected to potentially painful procedures Nonetheless, this area continues to be one of great debate, and no compromise between the sides ever seems 100% satisfactory. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Module 2.3 Measuring and Analyzing Results Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Figure 2.14 Why statistics can be misleading: Both of these graphs present the same data, an increase from 20 to 22 over 1 year’s time. But by ranging only from 20 to 22 (rather than from 0 to 22), graph (b) makes that increase look much more dramatic. (After Huff, 1954) Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Descriptive Statistics Descriptive statistics are mathematical summaries of results. There are two broad categories of descriptive statistics: Measurements of the Central Score Measurements of Variation or Dispersion Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Figure 2.15 Results of an imaginary survey of study habits at one college. This college apparently has two groups of students—those who study as hard as they can and those who find other things to do. In this case both the mean and the median are misleading. This distribution is bimodal; its two modes are 0 and 8. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Descriptive Statistics Measurements of the Central Score: The Mode The mode is the score that occurs most frequently in a distribution. The least useful of the three measures of central score, it comes in handy when a distribution is very abnormally distributed (when the majority of scores are clustered at the low end or high end) or when working with nonnumerical data (categorical variables such as diagnostic classifications.) Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Descriptive Statistics Measurements of the Central Score: The Mean The mean is the sum of all the scores divided by the total number of scores. This measure is most useful when the scores are normally distributed. A normal distribution, or normal curve, is a symmetrical frequency of scores clustered around the mean. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Descriptive Statistics Measurements of the Central Score: The Median The median is the middle score when we arrange all the scores in order from lowest to highest. It is especially useful when the scores we are working with are very abnormally distributed. For example, if our distribution of scores is 2, 3, and 10, 3 is a more accurate description of the “middle” then the mean, which would be 5 for this set of scores. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Concept Check: Calculate the mean, median and mode for this distribution of scores: 2, 3, 4, 4, 7, 10 Mean = 5 Median = 4 Mode = 4 Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Concept Check What would be the best measure of central score for this distribution? 1, 2, 2, 3, 3, 20 Median Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Concept Check What would be the best measure of central score for this distribution? 4, 4, 4, 4, 4, 4, 7, 8, 10 Mode Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Descriptive Statistics Measurements of Variation The range is a statement of the highest and lowest scores If our distribution has the following scores: 1, 2, 3, 5, 7, 9, 9, 10, the range is from 1 to 10. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Descriptive Statistics Measurements of Variation The standard deviation (SD) is a measurement of the amount of variation among scores in a normal distribution. The more closely the scores are clustered around the mean, the smaller the standard deviation is. Standard deviations are used to make meaningful comparisons on different tests or on different versions of the same kind of test. https://youtu.be/MRqtXL2WX2M Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Concept Check: On your first statistics exam of the semester, you get a score of 90, the mean for the class is 70 and the standard deviation is 20. On the second exam of the semester, you get an 80. The mean for the class is 65 and the standard deviation is 5. Did you do better, worse, or the same on the second test? You did much, much better on exam 2! Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Figure 2.18 In a normal distribution of scores, the amount of variation from the mean can be measured in standard deviations. In this example scores between 400 and 600 are said to be within 1 standard deviation from the mean; scores between 300 and 700 are within 2 standard deviations. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Evaluating Results: Inferential Statistics Probability Values A probability value is a way to estimate if a score would be extremely rare given what we know about the likely range in which the population mean falls. If the researcher says that there is a 95% certainty that the population mean falls between 5.0 and 7.0, and a particular score falls at 8.2, then that score has a probability value of less than 5% (p < .05) and it is an exceptional score in some way. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Evaluating Results: Inferential Statistics Probability Values and Statistical Significance Often scores that are exceptional in this way are interpreted as being unlikely to have arisen by chance. A result that is unlikely to have occurred by chance in a distribution is interpreted as being statistically reliable or statistically significant. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Figure 2.21 Researchers say that results are statistically significant if they calculate that chance variations in data would be unlikely to produce a difference between groups as large as the one that the researchers actually observed. Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Concept Check: Which is a more significant result: One that is obtained with a p-value of .10 OR One obtained with a p-value of .001? .001 Introduction to Psychology, 7th Edition, James W. Kalat Chapter 2: Scientific Methods in Psychology Statistics and Conclusions Consistent, dependable, large effects do not require statistics for analysis and interpretation. They speak for themselves. Psychologists are often dealing with small and fragile effects, or effects that only arise under a certain set of circumstances. To do meaningful work in this science, we need a solid understanding of research design and statistics.
