<unk> es (also known as<unk><unk>  analysis) form a class of research methods that involve observation of some subset of a  population of items all at the same time, in which, groups can be compared at different ages with respect of independent variables, such as IQ and memory. The fundamental difference between<unk><unk>  and longitudinal<unk> es is that<unk> es take place at a single point in time and that a longitudinal<unk>  involves a series of measurements taking over a period of time.<unk> es are used in most branches of science, in the social sciences and in other fields as well.
The alternative is longitudinal<unk> es.
Cross-sectional study
Longitudinal study
Cohort study
Randomized controlled trial
Clinical trial
Longitudinal studies
Placebo-controlled studies
Meta-analysis