The Theory of Planned Behaviour
Direct Measurement: Researchers typically use multi-item scales (e.g., semantic differentials or Likert scales) to assess the three main constructs directly. For example, asking how 'easy' or 'difficult' a task is to measure PBC.
Indirect (Belief-Based) Measurement: This involves identifying specific salient beliefs. Attitude is measured by multiplying the strength of a belief by its evaluation ().
Weighting the Predictors: The relative importance of attitude, subjective norm, and PBC in the prediction of intention is expected to vary across behaviors and situations. In some contexts, only attitude might be significant, while in others, PBC might be the dominant factor.
Predictive Modeling: The relationship is often expressed as a linear regression equation: where represents the empirically determined weights.
It is vital to distinguish between Perceived Behavioral Control and Actual Behavioral Control. While TPB focuses on the perception, the theory assumes that PBC serves as a proxy for actual control when the perception is accurate.
The following table compares TPB with its predecessor, the Theory of Reasoned Action (TRA):
| Feature | Theory of Reasoned Action (TRA) | Theory of Planned Behaviour (TPB) |
|---|---|---|
| Core Focus | Volitional behaviors (under total control) | Non-volitional and volitional behaviors |
| Key Predictors | Attitude and Subjective Norm | Attitude, Subjective Norm, and PBC |
| Direct Path | Intention Behavior | Intention Behavior AND PBC Behavior |
| Application | Simple choices (e.g., voting) | Complex goals (e.g., weight loss, quitting smoking) |
Identify the 'Control' Element: When analyzing a scenario, look for barriers like lack of money, time, or skill. If these are present, the question is likely testing your understanding of Perceived Behavioral Control.
Intention vs. Behavior: Always check if the question asks what a person plans to do (Intention) versus what they actually do (Behavior). TPB explains why these two are not always the same.
The Direct Path: Remember that PBC can predict behavior directly, bypassing intention. This happens when the behavior is constrained by external factors rather than just motivation.
Common Mistake: Do not confuse 'Subjective Norm' with 'Social Pressure' in a general sense. It must specifically relate to the individual's perception of what important others think they should do.
The 'Rationality' Assumption: A common misconception is that TPB assumes humans are purely rational. In reality, it assumes humans are 'reasoned'—they process information, even if that information is biased or incorrect.
Intention is not Action: Students often assume that a high intention score guarantees behavior. Factors like 'Actual Control' (e.g., a sudden illness or a broken car) can intervene, highlighting the gap between planning and doing.
Overlooking Background Factors: While TPB focuses on the three main constructs, it acknowledges that personality, culture, and demographics influence behavior indirectly by shaping the underlying beliefs.