1. Introduction

Population:

The complete collection of all individuals (Score, people, measurements) to be studied; the collection is complete, in the sense that it includes all individuals being studied.

Sample:

Sample is taken of the population or a subset of elements of the population selected according to some criterion. The infer certain for the characteristics of a population is possible if extract a representative sample. It is means

The sample dimension is determined of the minimum size need of the sample used in an experiment or study to present this scientific validity.

Generally, a sample size setting is related to a number of factors. The main ones are:

  1. Desired error in the conclusions about the experience.
  2. Risk of a mistaken conclusion.
  3. Predictability of the variables involved in the study.

Basically, there are two problems associated with obtaining samples: How to perform sampling. The determination of sample size.

2. Major sampling designs

  1. Simple Random Sampling:

n selected individuals so that each possible sample of the same size n has the same probability of being chosen

  1. Systemic Sampling:

The population elements are placed on a list and each list element is chosen (systematically) for inclusion in the sample. Choose a starting point and then select all k-th element in the population.

  1. Stratified Random Sampling:

Divides the population into at least two different subgroups which share the same characteristics, then perform a sample of each group.

  1. Cluster Sampling:

Divide the area into sections of the population (clusters or conglomerates); randomly pick some of these groups; choose all members of selected groups.

3. Experimental design

The quality of a scientific study is directly related to the care of researchers at the stage of experimental design.

Steps:

  1. Definition of the objectives and hypotheses; Objectives of a study corresponds to a question of scientific interest, should match always called question-problem. The definition of the hypotheses is due to the objective and the question-problem.

  2. Choice of factors (or variables); Identify the variables that must be selected.

  3. Study design; Definition of the type of study.

  4. Sample sizing; The definition of a sample size relating to the three mains factors mentioned in the above section.