This differs from stratified sampling in that in stratified sampling, all strata are sampled, whereas in cluster sampling we take a sample of clusters, not all clusters. Cluster sampling is used when it is difficult to construct a sampling frame for the entire population, or when it is too costly to visit randomly chosen population elements. A variable in a theory. Sometimes carries the connotation of something that cannot be observed directly, or which we suppose to exist but has not been measured yet. Similar in this sense to a latent variable.
Intelligence is a construct that is used to explain competence.
For example, if we want to examine the effects of age on attitude towards abortion, we collect attitude data from people of all ages, then see if there is a correlation between age and attitude. This is the opposite of a longitudinal study , where you take a set of young people, then measuring their attitude towards abortion every few years as they get older. The outcome of measurement . The set of values or codes that record what was observed, such as the blood pressure of 100 people. A descriptive study is similar to an exploratory study in that we do not attempt to test hypotheses.