In response to the increasing call by professional organizations, journal editors, and statisticians to include not only derived significance levels from quantitative statistical procedures but also measures of effect sizes (ES), this article first provides a rationale for school counseling-related researchers to include these key indices in their studies. We contend that with this information, readers are better able to contextualize and properly interpret research findings and conclusions. Second, the three major ES families and the indices associated with each one are reviewed. Next, the influence of various research designs and differing sample sizes on the size of these effects is discussed, followed by an introduction to ES computation, reporting, and interpretation, particularly as these issues relate to school counseling settings. Practical examples, ES summary tables, and supplementary resources are overviewed as well.
Get full access to this article
View all access options for this article.
References
1.
American Psychological Association. (2001). Publication manual of the American Psychological Association (5th ed.). Washington, DC: Author.
American School Counselor Association. (2005a). The ASCA national model: A framework for school counseling programs (2nd ed.). Alexandria, VA: Author.
4.
American School Counselor Association. (2005b). Foundations and basics.Alexandria, VA: Author.
5.
AstramovichR. L., CokerJ. K., & HoskinsW. J. (2005). Training school counselors in program evaluation.Professional School Counseling, 9, 49–54.
6.
BakerS. B., & GerlerE. R. (2004). School counseling in the twenty-first century (4th ed.). Upper Saddle River, NJ: Pearson.
7.
BaumanS., SiegelJ. T., DavisA., FalcoL. D., SeaboltK., & SzymanskiG. (2002). School counselors’ interest in professional literature and research.Professional School Counseling, 5, 346–352.
8.
BirdK. D. (2002). Confidence intervals for effect sizes in analysis of variance.Educational & Psychological Measurement, 62, 197–226.
9.
BrownD., & TrustyJ. (2005a). School counselors, comprehensive school counseling programs, and academic achievement: Are school counselors promising more than they can deliver?Professional School Counseling, 9, 1–8.
10.
BrownD., & TrustyJ. (2005b). The ASCA national model, accountability, and establishing causal links between school counselors’ activities and student outcomes: A reply to Sink.Professional School Counseling, 9, 13–15.
11.
CoeR. (2002, September). It's the effect size, stupid: What effect size is and why it is important. Paper presented at the annual conference of the British Educational Research Association, University of Exeter, England. Retrieved December 13, 2005, from http://www.leeds.ac.uk/educol/documents/00002182.htm
12.
CohenJ. (1990). Things I have learned (so far).American Psychologist, 45, 1304–1312.
13.
CohenJ. (1992). A power primer.Psychological Bulletin, 112, 155–159.
14.
FanX. (2001). Statistical significance and effect size in education research: Two sides of a coin.Journal of Educational Research, 94, 275–282.
15.
FidlerF. (2002). The fifth edition of the APA publication manual: Why its statistical recommendations are so controversial.Educational and Psychological Measurement, 62, 749–770.
16.
GreenS. B., & SalkindN. J. (2004). Using SPSS for Windows and Macintosh: Analyzing and understanding (4th ed.). Upper Saddle River, NJ: Prentice Hall.
17.
GrissonR. J., & KimJ. J. (2005). Effect sizes for research.Mahwah, NJ: Erlbaum.
18.
GysbersN. C., & HendersonP. (2006). Developing and managing your school guidance program (4th ed.). Alexandria, VA: American Counseling Association.
19.
HubertyC. J. (1987). On statistical testing.Educational Researcher, 16, 4–9.
20.
HubertyC. J. (2002). A history of effect size indices.Educational and Psychological Measurement, 62, 227–240.
21.
HubertyC.J., & LowmanL.L. (2000). Group overlap as a basis for effect size.Educational & Psychological Measurement, 60, 543–563.
22.
KirkR. E. (1996). Practical significance: A concept whose time has come.Educational & Psychological Measurement, 56, 746–759.
23.
KlineR. B. (2004). Beyond significance testing.Washington, DC: American Psychological Association.
24.
LapanR.T. (2001). Results-based comprehensive guidance and counseling programs: A framework for planning and evaluation.Professional School Counseling, 4, 289–299.
25.
LapanR. T. (2005). Evaluating school counseling programs. In SinkC. A. (Ed.), Contemporary school counseling: Theory, research, and practice (pp. 257–293). Boston: Houghton-Mifflin.
26.
MyrickR. D. (2003). Developmental guidance and counseling (4th ed.). Minneapolis, MN: Educational Media.
27.
O'GradyK. E. (1982). Measures of explained variance: Cautions and limitations.Psychological Bulletin, 92, 766–777.
28.
OlejnikS., & AlginaJ. (2000). Measures of effect size for comparative studies: Application, interpretations, and limitations.Contemporary Educational Psychology, 25, 241–286.
29.
RobertsJ. K., & HensonR. K. (2002). Correction for bias in estimating effect sizes.Educational and Psychological Measurement, 62, 241–253.
30.
RosenthalR., RosnowR. L., & RubinD. B. (2000). Contrasts and effect sizes in behavioral research: A correlational approach.Cambridge, UK: Cambridge University Press.
31.
RutherfordA. (2001). Introducing ANOVA and ANCOVA: A GLM approach.Thousand Oaks, CA: Sage.
32.
SchmidtJ. (2002). Counseling in schools: Essential services and comprehensive programs (4th ed.). Boston: Allyn and Bacon.
33.
SinkC. A. (2005). Comprehensive school counseling programs and academic achievement—A rejoinder to Brown and Trusty.Professional School Counseling, 9, 9–12.
34.
SnyderP., & LawsonS. (1993). Evaluating results using corrected and uncorrected effect size estimates.Journal of Experimental Education, 61, 334–349.
35.
SPSS. (2005). Data analysis with comprehensive statistics software. Retrieved December 10, 2005, from http://www.spss.com/spss/
36.
ThompsonB. (1990). Finding a correction for the sampling error in multivariate measures of relationship: A Monte Carlo study.Educational and Psychological Measurement, 50, 15–31.
ThompsonB. (1998). Statistical significance and effect size reporting: Portrait of a possible future.Research in the Schools, 5, 33–38.
39.
ThompsonB. (1999). If statistical significance tests are broken/misused, what practices should supplement or replace them?Theory & Psychology, 9, 165–181.
ThompsonB. (2002). “Statistical,” “practical,” and “clinical”: How many kinds of significance do counselors need to consider?Journal of Counseling & Development, 80, 64–71.
42.
ThompsonB. (2006). The role of effect sizes in contemporary research in counseling.Counseling and Values, 50, 176–186.
Vacha-HaaseT., & ThompsonB. (2004). How to estimate and interpret various effect sizes.Journal of Counseling and Psychology, 51, 473–481.
46.
VasquezL. M., GangsteadS. K., & HensonR. K. (2000, January). Understanding and interpreting effect size measures in general linear model analyses. Paper presented at the Annual Meeting of the Southwest Educational Research Association, Dallas, TX.
47.
WhistonS. C. (2003). Outcomes research on school counseling services. In ErfordB. T. (Ed.), Transforming the school counseling profession (pp. 435–447). Upper Saddle River, NJ: Merrill Prentice-Hall.
48.
WilkinsonL., & American Psychological Association Task Force on Statistical Inference. (1999). Statistical methods in psychology journals: Guidelines and explanations.American Psychologist, 54, 594–604.
