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RANDOM SAMPLING METHODS
Introduction
Many professions (business, government, engineering, science, social research, agriculture, etc.) seek the broadest possible factual basis for decision-making. In the absence of data on the subject, a decision taken is just like leaping into the dark. Sampling is of fundamental importance for estimating the uality of a lot or ascertaining its conformity to the reuirements of a specification. !he economy, reliability reliability and practicability of the sampling procedures have made them almost indispensable in most of the industrial and trade applications. "o#ever, "o#ever, the reliability of the conclusions dra#n on the basis of the sample depends on its representativeness and the method of its selection. It is hoped that the sampling methods as per standard, #hen implemented, #ould ensure a truly random and representative sample leading to sound and satisfactory estimation of lot uailty Sampling is a procedure, #here in a fraction of the data is taken from a large set of data, and the inference dra#n from the sample is e$tended to #hole group. !he surveyor%s (a person or a establishment establishment in charge charge of collecting collecting and recording data) or researchers researchers initial initial task is to formulate a rational justification for the use of sampling in his research. If sampling is found appropriate for a research, the researcher, then& (') Identifies the target population as precisely as possible, and in a #ay that makes sense in terms of the purpose of study. () uts together a list of the target target population from #hich the sample #ill #ill be selected. !his list is termed as a frame (more appropriately list frame) by many statisticians. (*) Selects the sample, and decide on a sampling sampling techniue, and+ () Makes an inference about the population. ll these four steps are inter#oven and cannot be considered isolated from one another. Simple random sampling, systematic sampling, stratified sampling fall into the category of simple sampling techniues. omple$ sampling techniues are used, only in the presence of large large e$peri e$perimen mental tal data data sets+ sets+ #hen #hen effic efficien iency cy is reuir reuired+ ed+ and, and, #hile #hile making making preci precise se estimates about relatively small groups #ithin large populations.
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SAMPLING TERMINOLOGY 1.population : population is a group of e$perimental data, persons, etc. (or) the population is a complete set of all possible observations of the type #hich is to be investigated.e. !otal number of students studying in a school or college, total number of books in a library, total number of houses in a village or to#n a. !inite population: population is said to be "inite if it consists of finite number of units.eg. /umber of #orkers in a factory, production of articles in a particular day for a company. b. In"inite Population: population is said to be in"inite if it has infinite number of units.eg. the number of stars in the sky, the number of people seeing the !elevision programmes. c. Population Total is the sum of all the elements in the sample frame. d. Population Mean is the average of all elements in a sample frame or population. #.Ele$ent: n element is an object on #hich a measurement is made. !his could be a voter in a precinct, a product as it comes off the assembly line, or a plant in a field that has either bloomed or not. %.&lu'ter. & population is built up of elementary units, #hich cannot be further decomposed. group of elementary units is called a cluster. (.Sa$plin !raction&!he fraction of the population or data selected in a sample is called the Sa$plin !raction.
Taret population is the population of ultimate clinical interest Taret population
Stud* population is subset of the target population to be studied
Stud* population
'a$ple
Sa$ple' are the subset of the study population used in clinical research because often not every member of the study population can be studied .
Ele$ent+ca'e
0.1 ai'in !actor & !he reciprocal of the sampling fraction is called the 1 aising Factor . ).Rando$ 'a$ple : sample, in #hich every unit has the same probability of selection, is called a random sample. !he #ord random describes the procedure used to select elements
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(participants, cars, test items) from a population.If no repetitions are allo#ed, it is termed as a simple random sample 'elected ,it-out replace$ent. If repetitions are permitted, the sample is 'elected ,it- replace$ent. .Sa$ple: Statisticians use the #ord 'a$ple to describe a portion chosen from the population. finite subset of statistical individuals defined in a population is called a sample. !he number of units in a sample is called the 'a$ple 'i/e. !he sample is referred to as representative because the characteristics of a properly dra#n sample represent the parent population in all #ays. 0.Sa$plin nit: !he constituents of a population #hich are individuals to be sampled from the population and cannot be further subdivided for the purpose of the sampling at a time are called sampling units.2g. to kno# the average income per family, the head of the family is a sampling unit. !o kno# the average yield of rice, each farm o#ner% s yield of rice is a sampling unit. 2.Sa$plin !ra$e: 3or adopting any sampling procedure it is essential to have a list identifying each sampling unit by a number. Such a list or map is called sampling frame. 2g. list of voters, a list of house holders, a list of villages in a district, a list of farmers etc. are a fe# e$amples of sampling frame. sample is a collection of sampling units dra#n from a frame or frames. 4ata are obtained from the sample and are used to describe characteristics of the population. E3a$ple 1 : Suppose #e are interested in #hat students in a particular high school think about the drilling for oil in our national #ildlife preserves. !he elements are the high school students and the population is the students #ho attend this high school. !he sampling units could be the students as individuals #ith the frame the alphabetical listing of all students enrolled in the school. !he sampling units could be homerooms, since each student has one and only one homeroom, and the frame the class list for homerooms. E3a$ple # : Suppose #e are interested in #hat voters in a particular precinct think about the drilling for oil in our national #ildlife preserves. !he elements are the registered voters in the precinct. !he population is the collection of registered voters. !he sampling units #ill likely be households in #hich there may be several registered voters. !he frame is a list of households in the precinct 14.Para$eter' And Stati'tic': 5e can describe samples and populations by using measures such as the mean, median, mode and standard deviation. 5hen these terms describe the characteristics of a population, they are called para$eter'. 5hen they describe the characteristics of a sample, they are called 'tati'tic'.
