Designing Experiments Flashcards
Experimental Methods
Experimental methods are the most rigorous way of testing hypotheses because they seek to establish cause and effect.
A true experiment has three key features:
1) Manipulation of the IV to produce a change in the DV.
2) Random allocation of participants to the different conditions.
3) Control of all the EVs so they remain constant.
The three features of true experiments are met to different degrees by different types of experiments:
1) Lab experiments (meets all three features).
2) Quasi-experiment (manipulation of IV and control of extraneous variables but not randomisation. Participants are allocated to conditions according to characteristics that they already possess).
3) Field experiment (controls of the IV but not the EVs, not necessarily random allocation).
4) Natural experiment (no manipulation of IV or EVs, not random allocation.
Experimental Design
In any basic experiment, there are two conditions; the experimental condition and the control condition.
Control condition is the condition where participants receive no experimental treatment. The group of participants who do not receive any experimental treatment are the control group and they act as a comparison to participants who do. A decision has to be taken as to whether participants will participate in one or both conditions. This will determine the experimental design.
Three basic experimental designs:
1) Independent groups design.
2) Repeated measures design.
3) Matched pairs design.
Independent groups design
Each participant takes part in one condition only. Participants are divided strictly on random basis. Should not be used unless there is some reason why repeated measures cannot be used.
Advantages of independent groups design
- Addresses order effects.
- Reduces demand characteristics.
- Can use the same stimulus.
Disadvantages of independent groups design
- Individual differences.
- Less effective at controlling participant variables.
- More participants needed.
Repeated measures design
All participants perform under both conditions of the experiment. This introduces a confounding variable; order effect. To prevent it, counterbalancing; half of participants perform condition 1 before condition 2 and the other half perform condition 2 before condition 1.
Advantages of repeated measures design
- Prevents individual differences.
- Fewer participants needed.
Disadvantages of repeated measures design
- Order effects.
- Demand characteristics.
- Can’t use the same stimulus.
Matched pairs design
Imitates repeated measures design but without each participant performing under both conditions. Participants are matched on all relevant variables, that is all variables that may affect the DV. Difficult to decide which variables are relevant. Ideally identical twins raised together.
Advantages of matched pairs design
- Prevents individual differences.
- Addresses order effects.
Disadvantages of matched pairs design
- Difficult to obtain/match - impractical.
- More participants needed.
