Experiments

Psychology AS · Research methodology. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

An experiment tests cause and effect. The researcher changes one thing, the independent variable (IV), measures its effect on the dependent variable (DV), and keeps everything else the same. A laboratory experiment takes place in an artificial, controlled setting. A field experiment takes place in the participants' normal environment, but the researcher still changes the IV.

The experimental design is how participants are placed in conditions: different people in each condition, the same people in every condition, or pairs of similar people split between conditions. Each design has its own problem, such as participant differences or order effects, and its own solution.

Rules to remember

Laboratory experiment: high control and a standard procedure, so reliable and easy to replicate; but artificial, so lower ecological validity and more demand characteristics.
Field experiment: natural behaviour and higher ecological validity; but less control of extraneous variables, and often no consent or debrief.
Independent measures: different participants in each condition. No order effects, but participant variables can affect results. Use random allocation.
Repeated measures: the same participants do all conditions. No participant variables, but order effects and demand characteristics. Use counterbalancing.
Matched pairs: participants are paired on a relevant variable and one of each pair goes into each condition. Fewer participant variables and no order effects, but matching is slow and never perfect.
Order effects: practice (getting better) and fatigue (getting tired or bored) in later conditions.
Counterbalancing: half do condition A then B, half do B then A. A control group or condition gives a baseline for comparison.

Common mistake

Students mix up random allocation with random sampling. Random allocation decides which condition each participant goes into; random sampling decides who takes part in the study at all.

Worked example

Twenty-four volunteers each complete a reaction-time test twice: once after a caffeine drink and once after water. Name the design, give one problem with it, and say how to deal with the problem.

The same people take part in both conditions, so the design is repeated measures.
Problem: order effects. They may be quicker the second time through practice, or slower through fatigue.
Solution: counterbalancing. 24 ÷ 2 = 12 have caffeine first and 12 have water first.
Order effects are then spread equally across both conditions.

Answer: Repeated measures design; order effects are the problem; counterbalance with 12 participants in each order.

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