Question

Detail, with very specific examples of each item, the meaning of the following concepts related to statistical data gathering: Be sure that the concept is first defined, IN YOUR OWN WORDS, and that you use a concrete example (including visual examples) to illustrate your understanding of the statistical concept: Simple Random Sample; Stratified Sample;Representative Sample; Convenience Sample; Observational Study

Answer 1

Simple Random Sample:- A sample obtained through the simple random sampling process. Simple Random Sampling widely known as SRS is most widely used probability sampling technique in research today. It is the technique of drawing a sample in such a way that each unit of the population has an equal and independent chance of being included in the sample. That is in a population of ‘N’ units the probability of drawing a unit is P(U)= 1/N. Selection of the sample can be made through lottery method or using a random table.

Stratified Sample:- Stratification means division into layers. Based on the character under study, the population is divided in to various groups (called strata) such that

• Units with in each group are as homogeneous as possible

• The group means are as wide as possible

Thus, In stratified sampling if ‘N’ is the total size of the universe, and we divide the whole population in to three stratums (in such a way that the units with in each strata are homogeneous) of sizes N1, N2 and N3 so that N1+N2+N3=N. A simple random sample is then chosen independently from each group (strata) of sizes n1, n2 and n3 out of N1, N2 and N3. The total sample ‘n’ out of the total population units ‘N’ is obtained by adding the three samples n1, n2, n3 drawn from each stratum of the same population. Sample size ‘n’ = n1+n2+n3 (Remember, population ‘N’ = N1+n2+N3)

Representative Sample:- A sample so selected that it truly represents the universe.

Convenience Sample:- In this method the sample is obtained by selecting ‘Convenient’ sampling units from the population. The method of convenience sampling is also called the chunk. A chunk refers to that fraction of the population being investigated which is selected neither by probability nor by judgment but by convenience. For example, a sample obtained from readily available list such as registration of cases in a hospital or from an institute, telephone directories etc.. is a convenience sample and not a random sample even if the sample is drawn at random from the lists. Convenience sampling is prone to bias as the sampling is not based on randomization. Hence, the results obtained by this method of sampling can hardly be representative of the population.