Sampling is the act, process, or technique of selecting a suitable sample, or a representative part of a population for the purpose of determining parameters or characteristics of the whole population.
REASONS FOR SAMPLING
There are six main reasons for sampling instead of doing a census. These are; * Economy
* The large size of many populations
* Inaccessibility of some of the population
* Destructiveness of the observation
* Accuracy or Reliability
The economic advantage of using a sample in research obviously, taking a sample requires fewer resources than a census. Unit cost of collecting data in the case of census is significantly less then in the case of sampling for example: In case of census is taka 200, while in the case of sampling is taka 1,000 but due to the larger number of items the total cost involve in the case of census of census is significantly higher then in the case of sampling. Timeliness
Unit time involve in the case of sampling then in the case census but due to the larger size of population total time involve in the case of census in significantly higher then in the case of census. Large size of many populations
In some cases the size of the population is extremely large. All of them are not treaseable due in traveling, disease, death, mental abnormality, prisoners etc. In that situation the only way to conduct the research is collecting data through a sample survey. Inaccessibility of some of the populations
In some cases the entire population may not be accessible. At that case sampling is necessary. Suppose in some cases the entire population is inaccessible because of aircraft crash. Destructiveness of the observations
Due to destructive nature of many of the population, the resources is completed to collect information only on a part of the population. For example:
Blood test for a patient.
Life hours of a tube light.
Accuracy or reliability
By using a scientific sampling technique one can minimize the sampling error and as qualified investigators are included, the non-sampling error committed in the case of sample survey is also minimum. The amount of non-sampling error in the case of census is much higher than the total amount of sampling and non-sampling error committed in the case of a sample survey ( as less qualified investigator are involve in the case of census and the supervision, monitoring and quality control mechanism in the case of census. The degree of errors has a relationship with reliability. If errors decrease than the reliability increase sampling decreases both the sampling and non-sampling error. So, it enhances the reliability of information. SAMPLING TECHNIQUES
The technique of selecting a sample is of fundamental importance in sampling theory and it depends upon the nature of investigation. The sampling procedures which are classified as: * Random
Within these types, you may then decide on a; point, line, area method. A. Random sampling
Least biased of all sampling techniques, there is no subjectivity - each member of the total population has an equal chance of being selected. Can be obtained using random number tables, Microsoft Excel has a function to produce random number. Methodology
* Random point sampling
* A grid is drawn over a map of the study area
* Random number tables are used to obtain coordinates/grid references for the points * Sampling takes place as feasibly close to these points as possible * Random line sampling
* Pairs of coordinates or grid references are obtained using random number tables, and marked on a map of the study area * These are joined to form lines to be sampled
* Random area sampling
* Random number tables generate coordinates or grid references which are used to mark the bottom left (south west) corner of quadrats or grid squares to be sampled. Advantages
* Can be used with...