WHAT ARE EXPERIMENTS?
The experiment is the most powerful method in the psychologist's toolkit because it is the only scientific technique for revealing causal relationships - the causes of human behaviour.
Right: stop there. It's a causal relationship, as in cause-and-effect.
It's not a casual relationship; a casual relationship is when he never calls you back.
These are the characteristics of experiments:
These are the characteristics of a true lab experiment but there are other types of experiment that don't have all of these features:
* Natural (or quasi-) experiments don't manipulate the IV; they observe changes in a naturally-occurring IV
** Field experiments don't take place in a controlled evironment
† Natural and field experiments cannot prove or disprove causation with the same confidence as a lab experiment
A lab experiment is the 'classic' experiment with all 5 features of a true experiment. It's strength comes from its "lab setting" which is a controlled environment.
A "lab setting" doesn't have to be a literal laboratory with test tubes and scientific gizmos.
No, a "lab setting" is any environment where the researcher is in control of everything that happens. So if you close your classroom door with a sign outside saying "DO NOT ENTER: EXPERIMENT IN PROGRESS", you've turned your classroom into a psychology lab.
Of course, there might still be a fire alarm or some other interruption. But in a lab setting, lots of extraneous variables are ruled out.
Another feature of a true lab experiment is that the IV is manipulated by the researcher. This level of control over all the variables is what makes the lab experiment so special. If you manipulate the IV and control all the other variables, then any changes in the DV must be caused by the IV. This is called internal validity: when you can be sure it is the IV affecting the DV and nothing else.
Despite this great advantage, there's a disadvantage to lab experiments. The artificial settings and tasks that give them such control can also make them unrealistic. Experiments whose results do not generalise to real life lack external validity, in particular they lack ecological validity if:
Beware. Just because many lab experiments lack ecological validity, it doesn't mean the ALL do. If you are criticising a lab experiment, it's not enough just to say "it lacks ecological validity because it's a lab experiment"; you have to point out something about the setting or the task that is artificial.
Another reason a lab experiment might lack external validity is because of demand characteristics. This is where participants try to figure out the purpose of the experiment they are in and stop acting naturally. Most lab experiments very obviously are experiments and the participants have been specially recruited to take part in them.
One way to improve the external validity of an experiment is to carry out the experiment in a real setting using real members of the public. This is a field experiment.
Field experiments are high in ecological validity because the setting is a real one and the task is usually something that would normally be done in that setting. For example, Sherif's Robbers Cave study observes boys forming teams and competing in a summer camp where such activities normally go on.
Field experiments may also be low in demand characteristics if the participants are not aware they are in an experiment and think the task they are doing is just part of normal life.
Remember that a field experiment is still an experiment. The researchers will be manipulating an IV and measuring a DV and trying to control as many extraneous variables as possible. If there's no IV, then it isn't a field experiment: it's just a naturalistic observation.
The disadvantage with field experiments is that the lack of control over the setting can introduce too many extraneous variables. For example, there may be interruptions, participants may leave, it will be hard for the researchers to observe everything that is going on or measure the DV accurately, especially if they are trying to do it in secret. If these variables interfere with the DV, then they are confounding variables and they lower the internal validity of the experiment.
For this reason, cause-and-effect conclusions from field experiments will always be a bit more tentative than field experiments; you cannot be so confident about accepting or rejecting the null hypothesis.
COMPARING LAB & FIELD EXPERIMENTS
If you are paying attention, you'll have noticed that lab experiments and field experiments are the opposites of each other: the strengths of one are the weaknesses of the other, and vice versa.
NATURAL (or QUASI-) EXPERIMENTS
Lab and field experiments are both "true experiments" because the researcher manipulates the IV. However, sometimes the researcher cannot manipulate the IV.
In these cases, the researcher has to observe changes in a naturally-occurring IV. For example, find people who are already left-handed or who are already drug addicts, then make comparisons.
An experiment with a naturally-occurring IV is a natural experiment.
Beware. You hear "natural experiment" and think that must be an experiment that takes place in a clearing in a forest with squirrels and birds... but no! The "natural" in "natural experiment" refers to the IV, not the setting. An experiment in a naturalistic setting is a field experiment.
Lovely. But NOT a natural experiment.
The biggest advantage of a natural experiment is that you get to do it in the first place: it's the only way of studying the effects of IVs it is impossible or unethical to manipulate.
Another advantage is high external validity: because the IV is one that comes from real life and hasn't been created deliberately by the researchers, you're more likely to be able to generalise the results to other real life groups and situations (other left-handed people, other drug addicts).
The disadvantage is massive. Because you're not manipulating the IV, you have to study the conditions of the IV as-and-where you an find them, with whatever left-handed people or drug addicts present themselves. This lowers internal validity immensely and makes it very hard to draw confident conclusions about cause-and-effect; you cannot accept or reject the null hypothesis with confidence.
For this reason, some people don't regard natural experiments as proper experiments at all. They call them quasi-experiments. "Quasi-" means "almost", so they are "almost experiments... but not quite".
Natural experiments can be in any setting. You can have a "natural experiment in a lab setting" or a "natural experiment in a field setting".
To make it more confusing, complex studies mix natural experiments with other sorts. For example, Bandura's Bobo Doll study manipulates some IVs (like the behaviour of the model) but also observes changes in naturally-occurring IVs (like the sex of the children); so it's partly a lab experiment, partly a natural experiment in a lab setting.
APPLYING EXPERIMENTAL METHODS IN PSYCHOLOGY