A statewide sample survey is to be made. First, the stateis subdivided into counties. Seven counties are selected atrandom and further sampling is concentrated on these sevencounties. What type of sampling is this?

A. Simple Random

B. Non-Proportional

C. Cluster

D. Stratified

Answer: Stratified random sampling

Step-by-step explanation:

In statistics, stratified sampling is defined as a method of sampling in which sampling is done from a population which can be partitioned into subgroups known as strata.

Example of a stratified sampling

In statistical surveys, when subgroup in an overall population differs, it maybe of great advantageous to sample each subgroup (stratum) independently. Stratification is the process of dividing members of the population into homogeneous subgroups or subpopulations before sampling is made. The strata should define a partition of the population. That is, it has to be collectively exhaustive and mutually: all element in the population can only be assigned to one and only one stratum. Then systematic sampling or the simple random sampling is applied within each stratum. The main aim is to increase the precision of the sample by minimizing sampling error.