Types of Experimental Design

Also called between groups or between samples.

P's are randomly allocated to one of two or more seperate groups. Random allocation means that every participant has an equal chance of being either in either group, for example drawing names out of a hat.

Each group of p's then only experience one condition of the investigation. This means that one group will have the independent variable, both groups then receive the dependent variable. A comparison of these figures will enable the researcher to say whether the IV has influenced the DV.

Independent groups:two or more groups of p's.

Problems!

May be important difs between groups to start with, and these rather than dependent variable may cause the results.

Requires more than p's than repeated measures design.

Advantages!

No associated problems with order effects as p's take part in investigation once.

This reduces demand characteristics as p's only experience one condition of the experiment.

Researcher does not lose p's between trials.

Also called matched participants.

In this design, p's are allocated to one of two (or more) groups as with independent group design. However, in this case it is not random allocation as each groups will have p's matching each other. For example, 2 male teenagers in each group. 

The investigation the proceeds in the same way as for independent measures design.

Matched Pairs: 2 matching groups of p's

Problems!

time consuming.

If one p drops out of it then a pair do.

nearly impossible to match pairs on every variable and any variable not matched could have impact on results. 

Advantages!

Eliminates problems associated with p variables as have matched each p with sim. person in dif group.

groups can be tested at same time or within short space of each other.

Also called within groups or within samples.

With this design, each participant carries out the same task under dif conditions. In other words they act as their own group. In one condition they recieve the independent variable and in the other condition they do not. They are then tested twice and their performance each time is measured (dependent variable). The scores are then compared by using a statistical test. 

Repeated Measures:a group of participants, used twice.

Problems!

Order effects-practise and fatigue. Can be solved by counterbalancing. 

Can't be used in compare and contrast studies. 

P's may guess the point of the study.

Advantages!

More accurate than the independent measures design as psychologist doesn't have to worry about individual difs. between p's.

requires fewer p's