Choosing a Design

1. Choosing a Design Pseudoexperimental and True Experimental Designs

2. Research Designs • Experimental designs are concerned with the manipulation of the IV and the measurement of its effect on the DV • Descriptive designs are concerned with the classification of the subjects and with the application of measurement procedures to subjects in order to assess group differences, developmental trends, or relationships among variables.

3. Research Designs • Both experimental and descriptive designs are classified as between-subjects, within-subjects, or mixed designs • the selection of a design depends largely on the research question

4. Between-Subject Designs • In experimental designs, different groups of Ss are exposed to different IVs • the IV is applied to one group (exp) but not to the other group (control) • The major threat to internal validity is subject selection • differences may be Ss selection differences rather than the IV • 2 ways to equate experimental and control groups: • randomization • matching

5. Between-Subject Designs • In descriptive designs, different groups of Ss are compared with each other with regard to their performance on some criterion variable • the important thing with descriptive designs is to select Ss who fall distinctly into the different categories of the classification variable but who are otherwise equivalent with regard to extraneous variables • classifications must be constructed that are mutually exclusive

6. Within-Subjects Designs • The performance of the same subjects is compared in different conditions. • In experimental research, the subjects are exposed to all treatment or levels of the IV • basic concern with these designs is that all conditions should be equivalent except for the application of the IV • therefore, it is important to control for sequencing or order effect

7. Within-Subjects Designs • There are 2 ways to control for the sequencing effect: • randomization • presentation of the experimental treatment conditions (IV) is randomly sequenced • counterbalancing • arrange all possible sequences of treatments (IV) and then randomly assign subjects to each sequence

8. Within-Subjects Designs • In descriptive research, longitudinal studies would be a within-subjects design

9. Mixed Designs • One IV may be studied with a between-subjects design while another IV is studied with a within-subjects design • this is a mixed design

10. Research Designs • Two ways to improve generalizability of findings with research designs: • random sampling of subjects • replication

11. Types of Experimental Designs • Pseudoexperimental designs (aka “pre-experimental”) • do not have built-in controls • there may be several explanations for the changes in the DV that are not solely caused by the IV • all of these designs have uncontrolled extraneous variables that threaten the internal validity of the experiment • weaker designs • True experimental designs

12. Types of Pseudoexperimental Designs • One-shot case study • One-group pretest-posttest • Static group comparison

13. One-shot case study • A descriptive study that has big weakness of no control • no comparisons can be made with this study • statistics: mainly descriptive statistics (e.g., means); some inferential statistics (e.g., correlation coefficients)

14. One-group pretest-posttest • Observations are made before and after the IV has been administered to the group • better than one shot case study, but still problem with 6 possible uncontrolled extraneous variables (history, maturation, testing, instrumentation, statistical regression, mortality) • statistics: parametric t-test for correlated samples; nonparametric sign test

15. Static group comparison • Comparisons are made between one group which is exposed to the IV and one group which is not • no pretest to compare posttest scores • no way to ensure equivalence between groups on relevant extraneous variables • problems may arise from subject selection variables, testing, instrumentation, mortality • statistics: parametric t-test; Mann-Whitney U test; chi-square

16. True Experimental Designs • Pretest-Posttest Control Group • Posttest-Only Control Group • Solomon Four-Group Design

17. Pretest-Posttest Control Group • 2 groups of Ss are compared on a measurement or observation on the DV. • Both groups are measured or observed twice • first measurement serves as the pretest and second measurement serves as the posttest • half of the Ss are randomly assigned to the first group while the second half are assigned to second group

18. Pretest-Posttest Control Group • Only threats to internal validity are testing and mortality • statistics: independent t-test to compare the two groups; analysis of covariance

19. Posttest-Only Control Group • Identical to the pretest-posttest control group design except that the pretest is not administered to either of the two groups • by random assignment of Ss to the two groups, it controls selection, history, maturation, and statistical regression • testing and instrumentation do no exist since none of the Ss is measured twice • mortality could be an internal validity problem

20. Solomon Four-Group Design • Subjects are randomly assigned to four different groups • two of the groups receive the treatment (IV) • two of the groups do not receive the treatment • only one of the control groups is administered the pre-test • all four groups are administered the posttest

21. Solomon Four-Group Design • It is a combination of the pretest-posttest and posttest-only control group designs • it controls for all threats to internal validity • statistics: 2-way ANOVA

22. What’s Coming Up …. • Human Subject Certification test (next week) • Group Work (next week) • Specific Group Designs (2 weeks) • Single-subject Designs (3 weeks) • Intro/Methods (Oct. 28) • JOURNAL CLUB (Nov 4) • Statistics • descriptive • inferential • Miscellaneous • interpreting research • role of research in CDIS

23. SLP Journal Club • 9 groups • 8 groups with 3 members • 1 group with 2 members • one group presentation/week • recommendation: select article based on your research project

24. AUD Journal Club • 2 groups • 2 groups with 2 members • one group presentation/week • recommend select article from research project