The population divided into a finite number of distinct and identifiable units is called sampling units. OR
The individuals whose characteristics are to be measured in the analysis are called elementary or sampling units. OR
Before selecting the sample, the population must be divided into parts called sampling units or simply sample units.
The list of all the sampling units with a proper identification (which represents the population to be covered is called sampling frame). The frame may consist of either a list of units or a map of area (in case sample of area is being taken), such that every element in the population belongs to one and only one unit.
The frame should be accurate, free from omission and duplication (overlapping), adequate, upto data and the units must cover the whole of the population and should be well identified.
In improving the sampling design, supplementary information for the field covered by the sampling frame may also be valuable.
Examples: Sampling Frame and Sampling Unit
- List of household (and persons) enumerated in population census.
- A map of areas of a country showing the boundaries of area units.
- In sampling an agricultural crop, the unit might be a field, a farm or an area of land whose shape and dimensions are at out disposal.
An ideal sampling frame will have the following qualities/characteristics:
- all sampling units have a logical and have numerical identifier
- all sampling units can be found i.e. contact information, map location or other relevant information about sampling units is present
- the frame is organized in a logical and systematic manner
- the sampling frame has some additional information about the units that allow the use of more advanced sampling frames
- every element of the population of interest is present in the frame
- every element of the population is present only once in the frame
- no elements from outside the population of interest are present in the frame
- the data is up-to-date
A sampling frame can be classified subject to several types of defect as follows:
A frame may be inaccurate: where some of the sampling units of the population are listed inaccurately or some units which do not actually exist are included in the list.
A frame may be inadequate: when it does not include all classes of the population which are to be taken the survey.
A frame may be incomplete: when some of the sampling units of the population are either completely omitted or includes more than once.
A frame may be out of date: when it has not been updated according to the demand of the occasion, although it was accurate, complete and adequate at the time of construction.