# 3 Data gathering and processing 3.5 Estimation

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As we now know, the goal of collecting data, including conducting surveys or acquiring data from other sources, is to obtain information about a particular population. When the sample has been selected and the information collected (see the data collection section) and data processed (see the data processing section), there still remains the task of linking the information gathered from the sample back to the target population.
Estimation is the process of finding an estimate (or approximation), which is a value derived from the best data available. Typically, estimation involves using the value derived from a sample to estimate the value of a corresponding population characteristic. Researchers are usually interested in looking at estimates of many statistics—totals, averages and proportions being the most frequent—for different variables. For example, a sample survey could be used to produce any of the following statistics:

• the proportion of smokers among all people aged 15 to 24 in the population;
• the average earnings of men and women with a university degree;
• the total number of cars possessed by the whole survey population.

In this section, we will outline what is the estimation process, starting with weighting followed by the estimation of sampling error and a description of the different sources of non-sampling error. Like sampling, estimation requires advanced knowledge of mathematical statistics. Examples presented in this section are based on the simplest sampling design, simple random sampling, and only aims to give an overview of the estimation process.

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