Preview
IB DP Psychology — Research Methodology | SL & HL | Paper 1 / Paper 3
🎯 Learning Intentions By the end of this note, you will be able to:
- Define reliability, validity, and credibility using IB-aligned terminology.
- Distinguish between internal and external validity, and between test-retest and inter-rater reliability.
- Explain how threats to reliability and validity affect the conclusions that can be drawn from psychological research.
- Evaluate research studies by applying reliability, validity, and credibility as analytical tools.
- Apply these concepts to a realistic case study scenario involving a psychological investigation.
- Analyse research using relevant concept areas: Measurement, Bias, Causality, and Responsibility.
- Discuss the relationship between reliability and validity, and why a study can be reliable without being valid.
Introduction & Context
Imagine you step on a bathroom scale every morning for a week. Each day it reads exactly 68 kg. You feel reassured — the scale is consistent. But then a friend points out that the scale was calibrated incorrectly and that your actual weight is closer to 72 kg. The scale was giving you the same answer every time, but the answer was wrong. This everyday frustration captures one of the most important tensions in psychological research: the difference between consistency and accuracy.
Psychologists face this challenge constantly. When a researcher designs a study to measure anxiety, aggression, or memory, they must ask two separate questions: Is my measurement producing consistent results? And is my measurement actually capturing what I claim it is capturing? These are not the same question, and confusing them leads to some of the most significant errors in psychological science. A third question then follows: even if the study is consistent and accurate, should we trust it — and for whom does it apply?
These three questions map directly onto the concepts of reliability, validity, and credibility. Understanding them is not just an academic exercise. They determine whether a psychological finding can be used to inform therapy, policy, education, or any real-world application. A study that fails on any of these dimensions may produce conclusions that are misleading, harmful, or simply irrelevant to the people it claims to describe.
🔍 Analogy — The Archer and the Target Imagine an archer shooting at a target. If all their arrows cluster tightly together in the top-left corner, far from the bullseye, the archer is reliable (consistent) but not valid (not hitting the intended mark). If the arrows are scattered randomly across the target, the archer is neither reliable nor valid. Only when the arrows cluster tightly around the bullseye is the archer both reliable and valid. Credibility then asks: was this a fair competition? Was the archer using the right equipment, under appropriate conditions, and would their performance generalise to other targets?
Core Concepts
Before any psychological finding can be taken seriously, the measurement process that produced it must be scrutinised. The first question to ask is whether the measurement is consistent — that is, whether repeating the same procedure under the same conditions produces the same result. This property is what psychologists call reliability, and it is the foundation upon which all other evaluations rest.
📖 Key Definition — Reliability Reliability refers to the consistency of a measurement or research finding. A measure is reliable if it produces the same or very similar results when repeated under the same conditions, or when assessed by different observers using the same criteria.
Reliability is not a single, unified concept — it has several distinct forms, each relevant to different aspects of