Experimental Methods:Statistics & Correlation

Mr. KochAP Psychology

Forest Lake High School

Statistics

• Types:– Descriptive – describe data– Inferential – mathematical procedures that help

psychologists make inferences about what data mean

• Distributions:– Frequency distribution– Histogram– Percentile rank

Central Tendencies

• Mean (M) – arithmetic average(1122333444455555) = 56/16 = 3.5

• Median – halfway point in a set of data(1122333444455555) = middle #(s)

• Mode – score that occurs most frequently5 = most frequent #

Variation

• Range = gap between highest and lowest scores (5-1)

• Standard Deviation (SD)– Calculate each difference (deviation) between each

score and the mean– Square these deviations– Find their average– Find the square root of this average– Use a “normal curve” to interpret results

Standard deviation gauges whether scores are packed together or dispersed because it uses information from each score. (The true basis for curving a test.)

M = 160/4 = 40 Sum of (deviations)² = 46

SD = √{[sum of (deviations)²]/number of scores} = √(46/4) = 3.4

Test Scores Deviation from the Mean (40)

Deviation Squared

36 -4 16

38 -2 4

41 +1 1

45 +5 25

The Normal (aka Bell) Curve

Negatively skewed Bell Curve Positively skewed Bell Curve

Inferential Statistics

• Statistical significance • Calculation of the likelihood a result happens by chance• Most use 5% (arbitrary) as standard for determining

significant difference between meansp < .05

• Principles:– Representative samples are better than biased samples– More cases are better than fewer– Less variable observations are more reliable than highly

variable observation

Correlation• Scatter plot• Correlation coefficient

• r = +1.00 One set of scores goes up in direct proportion to the other

• r = 0.00 Scores unrelated

• r = -1.00 One set of

scores goes up as the other goes down

Examples• + (positive)– Child abuse / aggressiveness– Education / income– Studying / grades

• - (negative)– Self esteem / depression– Age / hours of sleep– Stress / health

• Correlation DOES NOT prove causation

• Only proves relationship, not cause and effect

Correlation

• “Illusory Correlations”:– A perceived correlation that really does not exist• Ex: lucky penny = wishes come true