Parameters and Statistics

A statistic and a parameter are related concepts, often confused. A parameter is a measure of some characteristic of a population calculated from the whole population. A statistic is calculated from a sample of the population.
For example, we could find the IQ of every person in the United Kingdom, and then find the mean. This mean IQ would be a parameter> This particular parameter, the mean, is typically labelled  
To find the IQ of every person in the United Kingdom would take a lot of effort, money, contain errors and be very expensive. We can find a very good estimate of the average IQ in the UK by taking a suitable sample of people and finding the average IQ of that sample. The average IQ calculated in this way is called a statistic. This particular statistic, the mean is typically labelled  

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