Chapter ??

7

Statistical Issues Statistical Issues in Research Planning and in Research Planning and

EvaluationEvaluation

C H A P T E R

Chapter Outline

• Probability• Meaningfulness• Power• Using information in the context of the

study• Reporting statistical data

Interpreting Statistical Findings

• Probability– Alpha: level of chance occurrence (type I error)

• Typical: p < .05 or p < .01• Varying alpha• Truth table

(continued)

Interpreting Statistical Findings (continued)

• Exact probability– Beta (type II error)

• Meaningfulness (effect size)• Confidence intervals• Power: probability of rejecting the null

hypothesis when it is false

Truth Table for the Null Hypothesis

Sampling for Null Hypothesis (Figure 7.2)

Estimating Effect Size

• Effect size represents the standardized difference between two means.

• Formula: ES = (M1 – M2) / s• ES allows comparison between studies using

different dependent variables because it puts data in standard deviation units.

• An effect size of 0 is no difference, 0.2 is small, 0.5 medium, and 0.8 large.

Effect Size Examples of 0.5s (Figure 9.1a)

Effect Size Examples of 1.0s(Figure 9.1b)

Effect Size Curve to EstimateSample Size When = .05 (Figure 7.3)

Effect Size Curve to EstimateSample Size When = .01 (Figure 7.4)

Context of the Study

How do findings from the study fit within the context of

• Theory• Practice

Planning Research

Information needed in planning• Alpha• Effect size• Power• Sample size

Using the Power Calculator WhenReading a Research Study

When reading research, often sample size, means, and standard deviations are supplied. You can calculate the effect size by the formula in chapter 7. Using this data and the Power Calculator at the Web site below, you can estimate the power to detect a difference or relationship.

www.stat.ubc.ca/~rollin/stats/ssize/n2.html

Using the Power Calculatorto Plan Research

If you are planning your own research, you can often estimate the effect size from other studies. By setting your alpha (say .05) and power (say .8), you can use the Power Calculator at the website below to estimate the sample size you need.

http://calculators.stat.ucla.edu/powercalc/

Reporting Statistical Data(Summary From APA and APS)

• How was power analysis done?• Always report complications (screen your

data).• Select minimally statistical analyses.• Report p values of confidence intervals.• Report magnitudes of the effects.• Control multiple comparisons.• Report variability using standard deviations.• Report data to appropriate level.