What an experiment is

1. An experiment tests whether changing one variable produces a change in another.
2. The independent variable (IV) is the variable that is changed or compared between conditions.
3. The dependent variable (DV) is the variable that is measured to see the effect.
4. Experiments come in three types: laboratory, field and natural.
5. The three types are told apart by who changes the independent variable and by how ordinary the setting is.

Laboratory experiments

1. A laboratory experiment is run in a controlled setting.
2. The researcher manipulates the independent variable while holding other variables as steady as possible.
3. The setting can be any place where tight control is possible, such as a quiet classroom set aside for the study.
4. Every participant follows the same standardised procedure, so each one meets the same conditions.

Strengths and weaknesses of laboratory experiments

1. Strengths of a laboratory experiment.
   1. Extraneous variables are well controlled, so a change in the dependent variable can be put down to the independent variable with more confidence.
   2. The standardised procedure makes the study easy to repeat and check, so reliability is high.
   3. Equipment such as a stopwatch or a fixed word list can be used the same way for every participant.
2. Weaknesses of a laboratory experiment.
   1. The setting is artificial, so behaviour recorded there may not match what people do in ordinary life.
   2. Participants pick up demand characteristics from the cubicle, the equipment and the instructions, and change how they behave.

Ecological validity

1. Ecological validity is how far a finding applies to real, everyday settings.
2. A controlled setting strips out the noise, interruptions and choices that are present in ordinary life, which lowers ecological validity.
3. An artificial task lowers it as well, because recalling 20 unrelated words is not something anyone does outside a study.
4. A study raises its ecological validity by using a real setting and a task participants would carry out anyway.

Field experiments

1. A field experiment is carried out in a natural, everyday setting.
2. The researcher still manipulates the independent variable and still measures the dependent variable.
3. The participants are in a real situation, such as a bus, a high-street shop or a school corridor.

Strengths and weaknesses of field experiments

1. Strengths of a field experiment.
   1. Behaviour happens where it normally would, so ecological validity is high.
   2. Participants often do not know they are being studied, so they act naturally and demand characteristics are reduced.
2. Weaknesses of a field experiment.
   1. Extraneous variables such as the weather, the crowd or the time of day are hard to control, which weakens the claim about cause.
   2. The setting cannot be reproduced exactly, so the study is harder to repeat and reliability is lower.
   3. Informed consent is hard to gain when the people involved do not know they are taking part.

Natural experiments

1. In a natural experiment the independent variable varies on its own and the researcher records it.
2. The effect measured belongs to a change or difference the researcher did not create.
3. The independent variable is often a real-life event or an existing difference between people, such as which hand a person writes with.

Strengths and weaknesses of natural experiments

1. Strengths of a natural experiment.
   1. It allows the study of variables that could never be manipulated for practical or ethical reasons.
   2. The event or difference is real, so ecological validity is high.
2. Weaknesses of a natural experiment.
   1. The researcher has no control over the independent variable or over extraneous variables.
   2. The events or differences can be rare, so the study is hard to repeat and reliability is lower.
3. Because the independent variable was not manipulated, other differences between the groups could explain the results.
4. The two groups already exist, so their members differ in ways that were neither controlled nor measured.
5. A natural experiment therefore reports a link between the independent variable and the dependent variable.

Choosing the type of experiment

1. A laboratory experiment suits a question about a precise cause, such as whether a longer word list lowers recall.
   1. It gives tight control of variables and lets another researcher repeat the procedure exactly.
2. A field experiment suits a question about how people act in public, such as who offers a seat on a bus.
   1. It records everyday behaviour in the place where that behaviour normally happens.
3. A natural experiment suits a comparison of groups that already differ, such as children raised in two different care settings.
   1. It is the option available when the independent variable cannot be manipulated, either practically or ethically.

Turning this into marks

1. When a type and a reason are both asked for, keep them as two separate points, one naming the type and one naming what the setting does to the behaviour recorded.
2. Say who changed the independent variable, because "it was an experiment" does not secure the knowledge mark.
3. Tie ecological validity to the actual task, such as learning 20 unrelated words in a cubicle, rather than to the word artificial.
4. The mark most often lost is writing that a natural experiment proves a cause, when the groups already differed before the study began.