Snap shot study
A snap shot study is a study carried out over a very short period of time such as hours and days. In comparison a longitudinal study is carried out over a longer period of time such as weeks, months or years.
Snap shot studies are obviously quicker and cheaper to carry out than longitudinal approaches and rarely suffer from attrition. However they only provide us with a ‘snapshot’ of human behaviour and experience and therefore are not so useful when investigating development.

Experiment
An experiment is a research method used by psychologists which involves the manipulation of variables in order to discover cause and effect. It differs from non-experimental methods in that it involves the deliberate manipulation of one variable, while trying to keep all other variables constant.
The two main types of 'true' experiments are laboratory experiments and field experiments.
When psychologists carry out experiments they use one of three basic experimental designs to investigate the effects of an independent variable on a dependent variable. These are the independent measures design, the repeated measures design and the matched pairs design.

Independent Measures Design
If two groups in an experiment consist of different individuals then this is an independent measures design.
The main advantage of an independent measures design is that there is no problem with order effects.
However, the design also has disadvantages. The most serious is the potential for error resulting from individual differences between the groups of participants taking part in the different conditions.
Also if participants are in short supply, then an independent groups design may represent an uneconomic use of those available to participate, since twice as many participants are needed to obtain the same amount of data as would be required in a two-condition repeated measures design.