SAMPLE AND SAMPLE SIZE

SAMPLE AND SAMPLE SIZEPresented by : Roshni Maurya

What is the need for statistics?

Generalization of the results

Important statistical termsPopulation: a set which includes all measurements of interest to the researcher(The collection of all responses, measurements, or counts that are of interest)

Sample:A subset of the population

Target Population: The population to be studied/ to which the investigator wants to generalize his results.

Sampling Unit: Smallest unit from which sample can be selected.

Sampling frame List of all the sampling units from which sample is drawn.

Sampling schemeMethod of selecting sampling units from sampling frame.

Why sampling?Get information about large populationsLess costsLess field timeMore accuracy i.e. Can Do A Better Job of Data CollectionWhen its impossible to study the whole population

Sampling makes possible the study of a large, heterogeneous (different characteristics) population.

- The universe or population to be studied may be too large or unlimited that it is almost impossible to reach all of them. Sampling makes possible this kind of study because in sampling only a small portion of the population may be involved in the study, enabling the researcher to reach all through this small portion of the population.

PURPOSE & ADVANTAGE OF SAMPLE

Contd..Sampling is for economy.

- Research without sampling may be too costly. Sampling reduces the study population to a reasonable size that expenses are greatly reduced.

Sampling is for speed.

- Research without sampling might be too time consuming.

Sampling is for accuracy.

- If it takes too long a time to cover the whole study population, there may be inaccuracy. The research must be finished within a reasonable period of time so that the data are still true, valid and reasonable.

Contd

Contd.....Sampling saves the sources of data from being all consumed.- The act of gathering data may consume all the sources of information without sampling. In such a case, there is no more data to apply the conclusion to.

Disadvantages of Sampling (Defective Sampling)If sampling is biased, or not representative, or too small, the conclusion may not be valid and reliable.

In research, the respondents to a study must have a common characteristics which is the basis of the study.

Disadvantages of Sampling (Defective Sampling)If the population is very large and there are many sections and subsections, the sampling procedure becomes very complicated.

If the researcher does not possess the necessary skill and technical knowhow in sampling procedure.

WHAT IS A GOOD SAMPLE?The sample must be valid.

Validity depends on 2 considerations:

1. Accuracy bias is absent from the sample

2. Precision sample represents the population

SAMPLE SELECTIONActual sample selection can be accomplished in 2 basic ways:

Purposive selection

Random selection

SAMPLING DESIGNSDepending upon the type ; nature of population ; objectives of investigation; some commonly used designs are :Simple random sampling;Systematic random sampling;Stratified random sampling;Cluster sampling;Multiphase sampling;Pathfinder surveys;

SIMPLE RANDOM SAMPLING this type of sampling is one in which each and every unit in the population of the inquiry has an equal chance of being selected ; to be included in the sample.

Selection of unit is determined by chance only.

To ensure randomness, foll. methods are chosen: Lottery method. Table of Random Numbers.

Simple random sampling

Table of random numbers6 8 4 2 5 7 9 5 4 1 2 5 6 3 2 1 4 05 8 2 0 3 2 1 5 4 7 8 5 9 6 2 0 2 4 3 6 2 3 3 3 2 5 4 7 8 9 1 2 0 3 2 59 8 5 2 6 3 0 1 7 4 2 4 5 0 3 6 8 6

Contd.main advantage of this technique of sampling is that, it is easy to understand and it is easy to apply too, it assures randomness and eliminates personal bias.

disadvantage is that, it is hard to use with too large a population as selection may be widely spread that it enhances the cost and time to collect data.

SYSTEMATIC RANDOM SAMPLING

a technique of sampling in which every kth name in a list may be selected to be included in a sample.

also called as interval sampling, there is a gap or interval, between each selected unit in the sample.

Formed by selecting one unit at random and then selecting additional units at evenly spaced interval till sample of required size has been formed.

Sampling fraction Ratio between sample size and population size

This method may be selected to obtain a sample out of pts attending OPD of dental clinic. Systematic sampling

k ( skip interval )= population size sample size population size = 64 sample size = 8 k = 8

main advantage is that it is more convenient, faster, and more economical

disadvantage is that it can be adopted as long as there is no periodicity of occurrence of any particular event in population.

STRATIFIED RANDOM SAMPLING the process of selecting randomly, samples from the different strata of the population used in the study.

This means that a framework is laid down initially, and the subjects of study in a random sample are then allotted to compartments of framework.

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Ex: If it is decided to know prevalence of DMF teeth in diff. age grps, then the age grps form the strata and a random sample is to be chosen from each stratum i.e, age grp. ADVANTAGE: contributes much to the representative of the sample, provides greater accuracy, can conc. on wider geographical area. LIMITATION:Care of homogeneity in each stratum while dividing population into strata.

CLUSTER SAMPLING

used when the population is so big or the geographical area of the research is so large.(population forms natural grps or clusters like villages , wards block or children of school etc.)

First a sample of clusters is selected and then all units in each of selected clusters are surveyed.

Advantage : Efficiency; less time and cost.

Disadvantage: reduced accuracy or representativeness, on the account of the fact that in every stage there is a sampling error.

Cluster sampling

Section 4Section 5Section 3Section 2Section 1

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MULTIPHASE SAMPLING In this, a part of information is collected from the sample and a part from sub- sample.

Adopted when interest is in any specific disease.

Procedure is less costly; less laborious; more purposeful.

PATHFINDER SURVEYMethod used is a stratified cluster sampling technique ; includes most Population subgroups likely to have differing disease levels ; to cover a standard number of subjects in specific index age grp in any one location.

Method is suitable for foll. Situation :

Overall prevalence of various oral diseases affecting the population.

Imp. variations in disease level, severity and need for treatment in subgroups of population

To provide information about severity , progression of disease, to give an indication of increasing/ decreasing levels.

SAMPLE SIZE An optimum size of sample is to be considered on basis of foll. factors :

An approximate idea of estimate of characteristics under study and its variability from unit to unit in population.

Knowledge about the precision of the estimate of characteristic.

Probability level within which the desired precision is to be maintained.

Availability of exp. material , resources and other practical consideration.

Sample size is calculated by the formula:

n= Z2 * p * ( 1 p )

L2

CHARATERSTICS OF GOOD SAMPLE DESIGNMust result in a truly representative sampleMust result in small sampling errorsMust be viable in context of funds available for the research study.Must be such so that systematic bias can be controlled in a better way.

SAMPLING MODEL

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APPLICATION Evaluation of oral health status of a community.

Evaluation of health education on oral hygiene.

Studies on administrative aspects of services like availability and utilization of oral health facilitates in the community.

Advance data from surveys.

CONCLUSION Biostatistics as a discipline is concerned with inferring about the behaviour of a population given evidence from a sample. Depending on the objective of study any one of methods may be used to select sample . At times several methods of sampling may be used in the same study . Sample size has to be decided before selection of sample.

Thank you