Understand the reliability and validity of the following when analysing…

The idea

Psychologists evaluate reliability (consistency) and validity (accuracy) when planning research. Sampling methods affect population validity; random sampling is highly valid as it represents the target population, whereas opportunity sampling may lack validity due to bias. Experimental designs impact internal validity; repeated measures can reduce validity through order effects, but increase it by controlling participant variables. Quantitative methods (e.g., closed questionnaires) offer high reliability because they are easily replicated, but may lack ecological validity. Qualitative methods (e.g., unstructured interviews) provide rich data, increasing validity, but their subjective nature and lack of standardisation often reduce reliability.

What earns the marks

• Defining reliability as the consistency of a measure and validity as its accuracy.
• Evaluating how a specific sampling method impacts the population validity of a study.
• Explaining how quantitative methods improve reliability through standardisation and objective measurement.
• Discussing the trade-off between the high validity and low reliability often found in qualitative methods.

Common mistakes and how to put them right

• Using reliability and validity interchangeably.

  Put it right: Clearly distinguish reliability as consistency (getting the same result again) and validity as accuracy (measuring what is intended).

• Assuming quantitative data is always valid.

  Put it right: Recognise that while quantitative data is often reliable, it may lack ecological validity or fail to capture complex human behaviours.

• Stating that qualitative methods are completely invalid.

  Put it right: Explain that qualitative methods often have high validity because they provide in-depth, truthful insights, even if they lack reliability.

Exam tips

• Always define whether you are discussing internal validity (control of variables) or external validity (generalisability) when evaluating research procedures.
• When asked about reliability, mention standardisation and the ability to replicate the study to check for consistent results.
• Link the choice of sampling method directly to how well the findings can be generalised to the target population.