Study notes


  • Levels: AS, A Level
  • Exam boards: AQA, Edexcel, OCR, IB

Reliability is a measure of whether something stays the same, i.e. is consistent.The results of psychological investigations are said to be reliable if they are similar each time they are carried out using the same design, procedures and measurements.

Reliability can be split into two main branches: internal and external reliability.

Internal reliability

This describes the internal consistency of a measure (i.e. consistency within itself), such as whether the different questions (known as ‘items’) in a questionnaire are all measuring the same thing.

One way to assess this is by using the split-half method, where data collected is split randomly in half and compared, to see if results taken from each part of the measure are similar. It therefore follows that reliability can be improved if items that produce similar results are used.

External reliability

This assesses consistency when different measures of the same thing are compared, i.e. does one measure match up against other measures?

Discrepancies will consequently lower inter-observer reliability, e.g. results could change if one researcher conducts an interview differently to another.

Such reliability issues can be improved by standardising procedures (i.e. making sure that procedures are carried out the same way each time), for instance by implementing interviewer training, and/or practice through pilot studies.


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