Experiments, variables and hypotheses

Laboratory and field experiments

An experiment manipulates an independent variable to see its effect on a dependent variable, while controlling other variables.

• Laboratory experiment: in a controlled, artificial setting. High control, so cause and effect can be established and it is easy to replicate; but low ecological validity and more demand characteristics.
• Field experiment: in a natural setting, but the researcher still manipulates the IV. Higher ecological validity and participants may not know they are being studied (fewer demand characteristics); but less control over extraneous variables and ethical issues of consent.

(Natural experiments are not required in this unit.)

Variables and operationalisation

• Independent variable (IV): what the researcher changes (for example, whether a word list has short or long words).
• Dependent variable (DV): what is measured (number of words recalled in correct order).
• Operationalisation: defining variables precisely so they can be measured, for example 'number of words out of 20 written down correctly within 60 seconds'.
• Extraneous variables: other variables that might affect the DV. When an extraneous variable does affect the results it becomes a confounding variable, and we cannot tell whether the IV caused the change.

Hypotheses

• Experimental hypothesis: predicts that the IV will affect the DV.
• Null hypothesis: predicts no difference; any difference is due to chance.
• Directional (one-tailed): states the direction: 'Participants will recall more short words than long words.'
• Non-directional (two-tailed): 'There will be a difference in the number of words recalled from short and long word lists.'

Use a directional hypothesis when previous research (such as the word length effect) predicts the direction.

Worked example

Write an operationalised directional hypothesis for an experiment testing whether repeating 'la la la' affects recall of a word list.

1. IV: repeating 'la la la' (articulatory suppression) or recalling in silence.
2. DV: number of words out of 15 recalled correctly in 1 minute.
3. Direction: suppression reduces recall (it occupies the phonological loop).

Answer: Participants will recall fewer words out of 15 in one minute when repeating 'la la la' than when learning the list in silence.

Check yourself

1. A researcher compares recall of words learned in silence and with music playing. What is the IV?
   • The number of words recalled
   • The age of participants
   • Whether music is playing or not
   • The length of the words
   Show answer

   Whether music is playing or not. The IV is what the researcher changes.

2. In the same study, what is the DV?
   • The number of words recalled
   • Whether music is playing
   • The type of music
   • The room used
   Show answer

   The number of words recalled. The DV is what is measured.

3. What is the main strength of a field experiment compared with a laboratory experiment?
   • Greater control of extraneous variables
   • It is easier to get informed consent
   • Higher ecological validity because it takes place in a natural setting
   • It never involves an independent variable
   Show answer

   Higher ecological validity because it takes place in a natural setting. Natural settings make behaviour more realistic.

4. What is a confounding variable?
   • The variable the researcher manipulates
   • An extraneous variable that has affected the DV, so the cause of the result is unclear
   • The variable the researcher measures
   • A variable that has been fully controlled
   Show answer

   An extraneous variable that has affected the DV, so the cause of the result is unclear. Confounding variables make it impossible to be sure the IV caused the change.

5. 'Participants will recall more words in the morning than in the evening.' This is
   • a non-directional (two-tailed) hypothesis
   • a null hypothesis
   • an aim
   • a directional (one-tailed) hypothesis
   Show answer

   a directional (one-tailed) hypothesis. It predicts which condition will be higher.

6. Write a null hypothesis for an experiment comparing recall of short and long words.
   Show answer

   There will be no difference in the number of words recalled from short-word and long-word lists; any difference will be due to chance.. The null predicts no difference.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Psychology specification (issue January 2026)
• Pearson International A Level Psychology component guide: WPS01
• IAL Psychology revision site: Experimental methods
• IAL Psychology revision site: Variables and hypotheses

Last updated 8 November 2026. Spotted a mistake? Tell us.