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Scientific Method

� A set of procedures

� Test beliefs systematically

� Goal: reliable and valid information about the world

� Reject false claims

� Preserve reliable and valid claims – those we believe to be solidly established

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Steps of the procedure

� Formulate a hypothesis, or prediction� Precise

� Testable

� Gather data that address hypothesis� Run an experiment

� Collect observations

� Use already existing observations (other peoples’ records, such as those from a hospital, government agency, school system, etc.) 2

Reliability

� Consistency of results

– Do you get the same answer each time you ask the question?

– Temporal Stability� Degree of agreement between investigators

– Do you get the same answer when different people ask the same question?

– Interrater Reliability3

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External Validity

� The extent to which the data collected actually tells you something about the phenomenon that you are measuring.

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Other Issues to Consider

� Demand characteristics– Naturalistic Observation– Blind Conditions

� Observer bias– Double Blind

� Experimental control– Random Assignment

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Principles of Research Design

� Research design involves the plans for how a study is to be conducted

� Includes all of the following:– Variable

� Any characteristic that changes

– Population� The entire group of interest to a researcher

– Samples� A subset of a larger population

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Types of Research

� Descriptive studies

� Correlational studies

� Experimental studies

� Meta-analytical studies7

Descriptive Studies

� Descriptive designs� Study designs in

which the researcher defines a problem and variable of interest but makes no prediction and does not control or manipulate anything

� Do not involve testing hypotheses

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Descriptive Studies

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Longitudinal Studies

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Cross-Sectional Studies

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Correlational Studies

� Correlational designs� Studies that measure two

or more variables and examine their relation to each other

� Cannot be used to show cause-and-effect relationships

� Correlation does not equal Causation

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Positive Correlation

This scatterplot depicts a correlation coefficient of

r = +0.57

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Correlational Studies

� A correlation coefficient is a numerical representation of the relationship between two variables

� Can range from -1.00 to +1.00� +1.00 is a perfect direct relationship� -1.00 is a perfect inverse relationship

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Strength of Correlation

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Correlation does not Equal Causation� Sometimes we see causal relationships that

don’t actually exist.� Third variable problem� Third-variable correlation

– Matched Samples– Matched Pairs

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Third-Variable Problem: Matched Sample vs. Pairs

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Experimental Studies

� Independent variable

� Dependent variable

� Confounding variable

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Experimental Studies

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Experimental Studies

� An experimental study allows the experimenter to make direct inferences about the effect of one variable on another.

� The experimenter controls the presentation of one variable (the independent variable) and measures the other variable (the dependent variable).

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The effects of caffeine on reaction time

0

100

200

300

400

500

Nocaffeine

175 mg 350 mg 600 mg

Spee

d

The effect of caffeine on reaction time

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Experimental Studies

� Random assignment– Each participant in the study has the same chance of

being in an experimental or a control group

� Experimental group– The participants who receive the treatment being

investigated

� Control group– The participants who do not receive the treatment being

investigated

� Self-selection – A problem that occurs when anything about a person

determines whether he or she will be included in the experimental or control group 22

Manipulation

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Random Assignment

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Experimental Studies

� Placebo – A substance or treatment that appears

identical to the actual treatment but lacks the active substance

– Is sometimes given to the control group

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Experimental Studies

� Single-blind studies� Double-blind studies� Experimenter

expectancy effects– Robert Rosenthal and

the Discovery of Experimenter Bias

– Self-fulfilling prophecy

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Quasi-experimental Design

� Method similar to an experimental design except that it makes use of naturally occurring groups rather than randomly assigning subjects to groups

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Meta-Analysis

� A quantitative method for combining all published research results on one question and drawing a conclusion– Requires the use of effect sizes

� Standardized statistic� Allows different studies to be compared to each other

even if they used different methods or produced different statistics

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Meta-analysis of risk factors for PTSD in children and adolescents

Trickey et al., 201230