Jamil A. Malik (PhD)National Institute of Psychology

Quaid-e-Azam Univeristy

Categorical◦ Binary variable: Only two categories◦ Nominal variable: More than two categories

Ordinal variable:◦ Categories have a logical order

Continuous (entities get a distinct score):◦ Interval variable:

Equal intervals on the variable represent equal differences

◦ Ratio variable: Ratios of scores on the scale must also make

sense

correlational or cross-sectional research ◦ where we observe what naturally goes on in the

world without directly interfering with it

experimental research ◦ where we manipulate one variable to see its effect on

another◦ Tertium quid (confounding variables): A third person or

thing of indeterminate character

Causality and Statistics

Independent design

Repeated measure design

Systematic variation

Unsystematic (Random) variation◦ Randomization and counterbalancing

Frequency distributions (histogram)

Normal distribution

(1)Skew: lack of symmetry

(1)Positive

(2)Negative

(2)Kurtosis: pointyness

(1)Leptokurtic

(2)Platykurtic

In a normal distribution the values of skew and kurtosis are 0

The mode◦ Most frequent

The median◦ The Middle

The mean◦ The average

Range◦ Difference of Largest and smallest

Interquartile range◦ Range of middle 50%◦ Not affected by extreme scores

Frequency distribution as probability distribution◦ Normal distribution (z-score)◦ ±1.96 (2.5% of extreem scores in a distribution)◦ ±2.58 (1% of extreem scores in a distribution)◦ ±3.29 (0.1% extreem scores in a distribution)

Null hypothesis Alternate hypothesis

Degree to which a statistical model represents the data Sample and population

Real world models

Statistical Models

The mean Standard deviation and standard error Confidence intervals

One- and two-tailed tests

Type I and Type II errors

Effect sizes (d, r, eta sq, odd ratios)

r = .10 (small effect): In this case the effect explains 1% of the total variance.r = .30 (medium effect): The effect accounts for 9% of the total variance.r = .50 (large effect): The effect accounts for 25% of the variance.meta-analysis

Statistical power