EXPERIMENTAL, CORRELATIONAL, AND CLINICAL RESEARCH

• An experiment is an investigation seeking to understand relations of cause and effect.

• The manipulated variable is called the independent variable.

• The dependent variable is what is measured.

• The presence of the doll in both groups is the control variable, because it is constant in both groups.

• The researcher identifies a specific population, or group of interest, to be studied.

• Because the population may be too large to study effectively, a representative sample of the population may be drawn.

• Representativeness is the degree to which a sample reflects the diverse characteristics of the population that is being studied.

• Random sampling is a way of ensuring maximum representativeness.

  • Once sampling has been addressed, subjects are randomly assigned into both the experimental and control groups.

Sampling Bias

• The bias of selection from a specific real area occurs when people are selected in a physical space.

• Self-selection bias occurs when the people being studied have some control over whether or not to participate.

• Pre-screening or advertising bias occurs often in medical research; how volunteers are screened or where advertising is placed might skew the sample.

• Healthy user bias occurs when the study population tends to be in better shape than the general population.

• Single-blind design means that the subjects do not know whether they are in the control or experimental group.

• Double-blind studies are designed so that the experimenter does not inadvertently change the responses of the subject, such as by using a different tone of voice with members of the control group than with the experimental group.

• Correlational research involves assessing the degree of association between two or more variables or characteristics of interest that occur naturally.

• If an unknown factor is playing a role, it is known as a confounding variable, a third variable, or an extraneous variable.

• One way to gather information for correlational studies is through surveys.

• Clinical research often takes the form of case studies.

• Case studies are intensive psychological studies of single individuals.

Experimental Design

• Two important features of studies are the conceptual definition and the operational definition.

  • Whereas the conceptual definition is the theory or issue being studied, the operational definition refers to the way in which that theory or issue will be directly observed or measured in the study.

• Internal validity is the certainty with which the results of an experiment can be attributed to the manipulation of the independent variable rather than to some other, confounding variable.

• External validity is the extent to which the findings of a study can be generalized to other contexts in the “real world.”

• A related concept is inter-rater reliability, the degree to which different raters agree on their observations of the same data.

• STATISTICS : Descriptive statistics summarize data, whereas inferential statistics allow researchers to test hypotheses about data and determine how confident they can be in their inferences about the data.

Descriptive Statistics

• The mean is the arithmetic average of a set of numbers.

• The mode is the most frequently occurring value in the data set. (If two numbers both appear with the greatest frequency, the distribution is called bimodal.)

• The median is the number that falls exactly in the middle of a distribution of numbers.

• These statistics can be represented by a normal curve.

• The range is simply the largest number minus the smallest number.

• Variability refers to how much the numbers in the set differ from one another.

• The standard deviation measures a function of the average dispersion of numbers around the mean and is a commonly used measure of variability.

• Percentiles express the standing of one score relative to all other scores in a set of data.

• A positive skew means that most values are on the lower end, but there are some exceptionally large values.

• A negative skew means the opposite: most values are on the higher end, but there are some exceptionally small values. This creates a “tail” or skew toward the negative end.

• The correlation coefficient is a statistic that will give us such information.

• The Pearson correlation coefficient is a descriptive statistic that describes the linear relationship between two attributes.

Inferential Statistics

• Inferential statistics are used to determine our level of confidence in claiming that a given set of results would be extremely unlikely to occur if the result were only up to chance.

• Sample size refers to the number of observations or individuals measured.

• The null hypothesis states that a treatment had no effect in an experiment.

• The alternative hypothesis is that the treatment did have an effect.

• Alpha is the accepted probability that the result of an experiment can be attributed to chance rather than the manipulation of the independent variable.

• A Type I error refers to the conclusion that a difference exists when, in fact, this difference does not exist.

• A Type II error refers to the conclusion that there is no difference when, in fact, there is a difference.

  • The probability of making a Type I error is called the p-value.

ETHICS IN RESEARCH

• Occasionally, psychological experiments involve deception, which may be used if informing participants of the nature of the experiment might bias results.

  • This deception is typically small, but in rare instances it can be extreme.

• Stanley Milgram conducted obedience experiments in which he convinced participants that they were administering painful electric shocks to other participants, when, in fact, no shocks were given.

• Institutional Review Boards (IRBs) assess research plans before the research is approved to ensure that it meets all ethical standards.

• Participants must give informed consent; in other words, they agree to participate in the study only after they have been told what their participation entails.

• After the experiment is concluded, participants must receive a debriefing, in which they are told the exact purpose of their participation in the research and of any deception that may have been used in the process of experimentation.

• Confidentiality is another area of concern for psychology.

Next Chapter: Chapter 7: Biological Bases: The Brain and Nervous Systems