Research Methods
Lesson 7
Introduction to Experimental Design Methods and Techniques Produced by
2 x Adv Independent groups design 2 x Disadv
Experimental Design
2 x Adv Repeated measures design 2 x Disadv
2 x Adv Matched pairs design 2 x Disadv
Key Terms: Order Effects: • A confounding effect that can occur when a repeated measures design is used. • If participants always complete one condition first, by the time they get to the second condition they may experience order effects, such as practice, boredom and fatigue. • This could then affect their performance in the second condition.
Counterbalancing: • This is the method used to balance order effects in the repeated measures design. • Half the participants would complete the experiment in one sequence – for example Task A followed by Task B whilst the other group will complete Task B first and this will be followed by Task A.
Introduction to Experimental Designs • In a basic experiment there are two conditions: the experimental and control conditions. • A decision has to be taken as to whether a participant will take part in one or both conditions. • The decision taken will determine the experimental design. • The experimenter can choose from the following designs: independent groups design, repeated groups design and matched pairs design.
Independent Groups Design • In an independent groups design the participants take part in either the control or the experimental condition. When should it be used: • When it is suitable for the proposed experiment, this is when participants do not need to be matched very closely. • A large sample is needed for this design and participants need to be randomly allocated to the conditions otherwise individual differences are likely to be confounding variable.
Advantages of the independent groups design: • No order or practice effects, by this we mean the participants performance in either of the groups is not influenced in anyway by previous knowledge as a result of earlier participation. • This will automatically reduce any demand characteristics. • The experimenter can use the same stimulus material for both conditions.
Disadvantages: • Least effective way for controlling of participants variables. • More participants are needed than the repeated measures design.
Repeated Measures Design • In a repeated measure design every participant will take part in both conditions. In effect the participant will serve as their own control. • If we were investigating the effects of alcohol response times, the participant will participate in the condition with and without alcohol. When should it be used: • When there is only a small number of participants and the design of the experiment allows for the same participants to be used in both the conditions.
Advantages: • Participant variables are eliminated. • Identifying participants for a research can be difficult, this design requires fewer participants. Disadvantages: • Order or practice effects do occur, e.g. fatigue, learning, and boredom can impact performance usually in the second condition. • Increased chance of demand characteristics. • The same stimulus material cannot be used twice.
Controls: • Order effects can be controlled through counterbalancing. • Chances of demand characteristics preferably use a single blind fold.
Matched Pairs Design • In this design each participant in one of the conditions is matched as closely as possible with a participant in the other condition. • Examples of variables that could be matched include age, gender, intelligence and personality traits. • When the matching pairs have been established they are randomly allocated to either one of the conditions, that is experimental or control.
When should it be used: • When the researcher or investigator has a lot of money to invest in the experiment as participants are difficult to find and need to be carefully and appropriately matched.
Advantages: • No order or practice effects. • Good attempt at controlling participant variables. Disadvantage: • Difficult to match participants exactly. • More participants are needed.
Controls: • Monozygotic twins (identical) provide researchers with a very close match for participant variables.
Examiners Tips • You need to know at least two advantages and disadvantages of each design. • Just stating the advantage or disadvantage will not earn you the full marks. You need to explain what is meant by that advantage or disadvantage. • For example, the one disadvantage of the repeated measures is order effects (e.g. practice, fatigue and boredom could influence the outcome. If participants complete the tasks in the same order, then performance in the second condition can be significantly affected.
Key Points • Repeated measures design uses the same participants in both conditions. • Independent groups design is where the participants are randomly allocated to either one condition or the other. • Matched pairs design is where each participant is matched with another participant on relevant variables. The pairs are then randomly allocated to either one or the other condition.
• Each of the above have advantages and disadvantages – each design can be considered in terms of their ability to control participant variables, the number of participants required, vulnerability to demand characteristics and whether the same stimulus materials can be used.