- In a survey we gather data on existing conditions, attitude, or behaviors by ratings in a form of questionnaire or direct observations, and demographic information such as age, sex, income.
- In an observational study data are collected via ``observation'' based on the existing conditions which are not controlled in the sense of proper scientific experiment.
- In a scientific study (or an experimental study) an experimenter controls ``experiments'' and various conditions of measurement (e.g., temperature and other quantitative qualities).
A data set is a collection of observations on one or more variables recorded as an outcome of experiment or survey. It is very important to envisage a population from which the data are drawn, and to consider the data as a random sample which is representative of the underlying population. Each variable possesses a different characteristic such as household income, metal temperature, cell diameter, seed type, fertilizer level, and so on.
- A variable is called categorical or qualitative when data are recorded as several categories. For example we may observe the different types such as ``round yellow'', ``wrinkled yellow'', ``round green'', or ``wrinkled green'' in the seed type of progeny in plant breeding.
- A variable is called quantitative or numerical when data are numerical values such as 0.223 or 152.7.
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