experimental design

What is a matched pairs design?

Participants only do one experimental condition, but all participants are first matched and put into pairs based on important characteristics

Steps of a matched pairs design

1. Identify potential extraneous variables

2. Screen people for this extraneous variable

3. Put participants in order (e.g. age)

4. Pair in order (e.g. 2 oldest paired, next 2 oldest paired)

5. One participant from each pair randomly allocated to each condition

Strengths of matched pairs design x3

An attempt has been made to control participant extraneous variables - they are first out into pairs so participant extraneous variables are reduced

Fewer demand characteristics - only exposed to single condition

No order effects - each participant only takes part in one condition

Weaknesses of matched pairs x2

Less economical - takes time and effort to first screen people, match them and allocate them

Participant extraneous variable - it is impossible to match participants on every single relevant characteristic

What are order effects?

A consequences of partipants doing all conditions so the order might affect their performance in the second condition

What is the practice effect?

Participants may become practiced at the test so their performance will improve

What is the fatigue effect?

Participants may become tired of the test so their performance deteriorates

what is an independent groups design?

Where participants only do one condition of the independent variable.

Strengths of the independent groups design x2

No order effects - participants only take part once so won’t display practice effects so increased internal validity

Fewer demand characteristics - participants only take part once so are less able to guess the aim of the study.

Weaknesses of independent group design x2

Less economical - twice the number of participants are needed compared to repeated measures so likely to take longer and therefore cost more (time is money)

Participants extraneous variables- different participants in each condition so findings could be due to characteristics not independent variables.

What is the repeated measures design?

Where participants do all experimental conditions

Strengths of repeated measures design x2

Participant extraneous variables are controlled- all participants in each condition are the same so there won’t be individual characteristics so confident internal validity.

More economical - half the number of participants required so each participant does both conditions and therefore quicker so costs less.

Weaknesses of repeated measures design x2

Order effects - participants take part in both conditions so manipulation of independent variable may be better or worse.

Demand characteristics - participants complete all conditions so may guess aim and change behaviour to please or sabotage (screw you effect) the experiment.