This technique is also referred to as double sampling. Double sampling is particularly useful in situations in which the enumeration of the character under study (main character) involves much cost or labour whereas an auxiliary character correlated with the main character can be easily observed. For example, in estimating the volume of a forest stand, we may use diameter or girth of trees and height as auxiliary variables. Lecture Number 4 Introduction to Sampling Methods October 28, 2016 27 / 35
Probability Proportional to Size Sampling In many instances, the sampling units vary considerably in size and simple random sampling may not be effective in such cases as it does not take into account the possible importance of the larger units in the population. In such cases, it has been found that extra information about the size of the units can be gainfully utilised in selecting the sample so as to get a more efficient estimator of the population parameters. One such method is to assign unequal probabilities for selection to different units of the population. For example, villages with larger geographical area are likely to have larger area under food crops and in estimating the production, it would be desirable to adopt a sampling scheme in which villages are selected with probability proportional to geographical area. Lecture Number 4 Introduction to Sampling Methods October 28, 2016 28 / 35
Probability Proportional to Size Sampling When units vary in their size and the variable under study is directly related with the size of the unit, the probabilities may be assigned proportional to the size of the unit. This type of sampling where the probability of selection is proportion to the size of the unit is known as ‘PPS Sampling’. Lecture Number 4 Introduction to Sampling Methods October 28, 2016 29 / 35
Convenient Sampling A type of nonprobability sampling which involves the sample being drawn from that part of the population which is close to hand or readily available and convenient. It is sometimes known as grab or opportunity sampling or accidental or haphazard sampling. A research team may determine that the most convenient and economical method is to set up an interviewing booth from which to intercept consumers at a shopping center. The researcher using such a sample cannot scientifically make generalizations about the total population from this sample because it would not be representative enough. Convenience samples are best used for pilot testing or exploratory research when additional research will subsequently be conducted with a probability sample. Lecture Number 4 Introduction to Sampling Methods October 28, 2016 30 / 35
Judgement Sampling A nonprobability sampling technique in which an experienced individual selects the sample based on his or her judgment about some appropriate characteristics required of the sample member. The researcher chooses the sample based on who they think would be appropriate for the study. Researchers select samples that satisfy their specific purposes, even if they are not fully representative.
- Fall '18
- F. TAILOKA
- representative, Basic Concepts of Sampling