Standardizing Variables in Psychological Statistics

Standardizing Variables

• Module 5 focuses on standardizing variables in psychological statistics.

Key Topics

• Standardized scales, z-Scores, effect size, study questions, and analyses using jamovi and RStudio.

Standardized Scores

• Physical variables have natural metrics; psychological variables use standard scores for universal interpretation.

Common Standardized Scales

• IQ Tests: Mean of 100, SD of 15.

• GRE Subtests: Mean of 150, SD of 9.

• Personality Inventories: T score scale with mean of 50, SD of 10.

T-Scores

• Indicate a score's location in a distribution, facilitating comparisons across different metrics.

MMPI

• Converts sum scores to T scores for personality and psychopathology assessment.

z-Scores

• Mean of 0, SD of 1.

T-Scores vs. z-Scores

• T-scores used when population mean and SD are unknown or N < 30.

CES Depression Scale

• 20-item questionnaire; scores can be converted to z-scores for universal metric.

z-Score Formula

• z = \frac{X - \mu}{\sigma}; converts raw score distance from mean to standard deviation units.

T-Scores Transformation

• T = 10z + 50

Rule of Thumb

• 95% of scores are within ± 2 SD from the mean in a normal curve.

Uveal Melanoma and Depression Study

• Examined visual impairment and optimism/pessimism on depressive symptoms.

Key Variables

• Cancer Diagnosis and Depression (measured by CES-D).

Standardized Mean Difference (Cohen’s d)

• Expresses mean difference in standard deviation units: d = \frac{\mu1 - \mu2}{s_p}

Effect Size Interpretation

• Guidelines: Negligible (< |0.20|), Small (|0.20 to 0.50|), Moderate (|0.50 to 0.80|), Large (> |0.80|).

Study Questions

• Includes