Quantitative Research Foundations: Variables, Theories, Hypotheses, Validity, Bias, Sampling, and Measurement

Concept

abstract idea (e.g., "weight" or "moderation").

Construct

more complex (e.g., "diet quality," "mindful eating").

Variable

measurable version of a construct (e.g., BMI, cholesterol level).

Operational definition

specific method used to measure a variable (e.g., Healthy Eating Index for diet quality).

Independent variable

manipulated by the researcher (cause).

Dependent variable

measured outcome (effect).

Extraneous/confounding variables

outside influences that can distort results.

Covariates

variables statistically controlled for (like age or gender).

Levels of Measurement

Ratio (true zero, e.g., weight), Interval (equal spacing, no true zero), Ordinal (ranked), Nominal (categories).

Theories

explain relationships among variables (e.g., Social Cognitive Theory).

Models

visually represent theoretical ideas (e.g., Transtheoretical Model: stages of change).

Frameworks

guide study design and are less formal than theories.

Hypothesis

An educated guess predicting relationships between variables.

Associative hypothesis

variables are correlated but not causal.

Causal hypothesis

independent variable causes changes in dependent variable.

Directional hypothesis

prediction specifies direction of effect.

Nondirectional hypothesis

prediction does not specify direction of effect.

Null hypothesis (H₀)

assumes no effect; tested statistically.

Research hypothesis (H₁)

predicts an effect.

Statistical significance (p ≤ 0.05)

results unlikely due to chance.

Effect size

shows magnitude or strength of effect (important for practical/clinical relevance).

Type I error (α)

false positive — reject true null.

Type II error (β)

false negative — accept false null.

Reliability

consistency or repeatability of measurements.

Validity

accuracy — whether the study measures what it claims.

Internal validity

independent variable truly causes effect.

External validity

results can be generalized to other settings.

Statistical conclusion validity

accuracy of statistical inferences.

Construct validity

accuracy in measuring theoretical concepts.