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PSYC512: Research Methods Lecture 15

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PSYC512: Research MethodsLecture 15

Brian P. Dyre

University of Idaho

PSYC512: Research Methods

- Review of Lecture 14
- Multi-factor research designs

- Today: other specialized research designs
- Statistical “nuts and bolts” in conducting hypothesis tests of means
- Combining experimental and correlational designs (Analysis of covariance or ANCOVA)
- Quasi-Experimental Designs
- Developmental Designs
- Small-n designs and Psychophysical Methods

PSYC512: Research Methods

- The central limit theorem
- The mean of the sampling distribution of means is equal to the mean of the population from which the samples were drawn.
- The variance of the sampling distribution of means is equal to the variance of the population from which the samples were drawn divided by the size of the samples. Expressed as the standard deviation:
- If the original population is distributed normally (i.e. it is bell shaped), the sampling distribution of means will also be normal. If the original population is not normally distributed, the sampling distribution of means will increasingly approximate a normal distribution as sample size increases. (i.e. when increasingly large samples are drawn)

PSYC512: Research Methods

- s of population is known
- z test of sample mean

- s of population is estimated as s
- t-test of sample mean
- As N approaches infinity, t approaches z
- For small N distribution of sample variance is skewed
- Df = N-1

PSYC512: Research Methods

- Differences in means of matched samples
- Differences in means of Independent samples

PSYC512: Research Methods

- Combining between-subjects and within-subjects factors in research design – mixed designs
- Combining experimental and correlational designs (Analysis of covariance or ANCOVA)
- Quasi-Experimental Designs
- Pretest-posttest designs
- Developmental designs (Longitudinal or cross-sectional)

PSYC512: Research Methods

- Covariates in experimental designs
- Measure your subjects on a covariate—a variable that you believe may be correlated with your dependent variable
- If left unmeasured these covariates add error variance and might obscure significant effects
- Measuring the covariate allows you to use correlational statistical techniques in your analysis (e.g., Analysis of Covariance or ANCOVA) to “subtract out” the error variance associated with the covariate, thereby increasing the statistical power of your experiment
- Example: measuring IQ in a learning experiment

PSYC512: Research Methods

- Quasi-independent variable in experimental designs
- “Quasi” means “kind of, but not really”
- Similar to including a covariate, except
- measurement of covariate is used to assign Ss to groups
- Covariate is thus treated as an quasi-independent variable

- Quasi-independent variables are referred to as “quasi” because they cannot be manipulated, they are essentially dependent variables (measures) that are treated as independent variables in the experimental design and analysis

PSYC512: Research Methods

- Quasi-experimental designs are those in which only quasi-independent variables are used
- Time series vs. pretest-posttest designs
- Time series: Measure behavior several times prior to and following a treatment (time series design) or change in your quasi-independent variable (interrupted time series design)
- Pretest-posttest: Measure behavior once prior to and once following the change in your independent variable

PSYC512: Research Methods

- Equivalent time samples design
- Time-series design especially useful for treatments with transient effects
- Repeatedly measure behavior following multiple applications and withdrawals of the treatment

- Non-equivalent control group designs – helps control for history confounds which should affect both groups equally

PSYC512: Research Methods

- Used to assess changes in behavior related to a person’s chronological age, which serves as a quasi-independent variable
- Cross-sectional designs
- Simultaneously test subjects assigned to two or more age groups
- Generational effects can confound the age variable

- Longitudinal designs
- Repeatedly test a single group of subjects over time
- Controls for generational effects—but, may still limit external validity
- May be confounded by history, mortality, and/or multiple observation effects

- Cohort-sequential design
- Combines longitudinal and cross-sectional designs by measuring multiple age groups over time which allows evaluation of generational or historical confounds

PSYC512: Research Methods

- Research that focuses on identifying functional relationships between variables and performance of a single subject (e.g., behavioral analysis and psychophysics)
- Typically involve
- Large number of observations
- Rigid experimental control
- Investigations of powerful variables whose effects are easily detected

PSYC512: Research Methods

- Same as time-series design
- Time series: Measure behavior several times prior to and following a treatment

- Two phases (A and B)
- A: baseline phase to establish behavioral baseline performance on DV prior to treatment, requires that a stability criterion be reached
- B: intervention phase that measures performance on DV after treatment

PSYC512: Research Methods

- Problem: Time confound
- Solution: ABA Design—reverse the treatment by removing it and see if performance returns to baseline (unlikely to occur by coincidence)
- Problem with reversal: now behavior is at baseline again
- Solution: ABAB design

A B A B

PSYC512: Research Methods

- Used to assess irreversible changes in behavior
- Assess multiple independent behaviors and introduce treatment to only one behavior at a time
- Controls for time effects (history, maturation)

PSYC512: Research Methods

- Used to determine thresholds and difference thresholds (just-noticeable differences or JNDs)
- Ss receive dozens or hundreds of trials under tightly controlled conditions
- Methods
- Method of Constant Stimuli
- Method of Adjustment
- Method of Limits

PSYC512: Research Methods

Predicted by

absolute threshold

- Fechner’s Elements of Psychophysics (1860)
- Absolute threshold (limen): how much energy must exist in a stimulus for it to be detectable?

1.0

Distribution of “absolute” threshold

Probability of Detection

“Real World”

0.5

Amount of Stimulus Energy

Subliminal: below threshold

Superliminal: above threshold

PSYC512: Research Methods

- Method of adjustment
- Intensity or feature of stimulus is changed until it matches a standard
- Hysteresis requires both ascending and descending trials
- Average match across ascending and descending determines threshold
- Fast, but least accurate

PSYC512: Research Methods

- Method of Limits (discrete method of adjustment)
- Like method of adjustment except adjustment is done in discrete steps whose size is controlled by the experimenter
- Hysteresis requires both ascending and descending trials
- Average match across ascending and descending determines threshold
- Variant: Staircase method

PSYC512: Research Methods

- Method of Constant Stimuli
- Choose 5-9 stimuli, some above, some below threshold
- Present in random order
- threshold equals amount of stimulus energy that detected 50% of the time
- Slowest but most accurate

1.0

Probability of Detection

0.5

Threshold

Amount of Stimulus Energy

For all Methods: sensitivity = 1/threshold

PSYC512: Research Methods

- On-campus (“Live”) students: please e-mail me article presentation time slot preferences

PSYC512: Research Methods