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When do we calculate the population parameter instead of sample statistics?

If there is a this kind of case which statistical tool should we use population parameter or sample statistics?

A car company wants to know the average age of cars of their brand that are still on the road. How would you define the appropriate population? Will the car company calculate a population parameter or sample statistic? Why?

kjetil b halvorsen
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Muhammet Turkmenoglu
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    How often do you have the entire population of interest? How often is it even possible? – Glen_b Oct 07 '13 at 12:56
  • @Glen_b There's some risk that your comment could be misunderstood, depending on how "have" is interpreted. In the example of this question, a population is defined fairly clearly: the set of cars made by the company that are "still on the road" at some specified time. If "on the road" and that time are defined sufficiently well, there is little doubt what the population is and in some sense we "have" it. The second part of your comment perhaps addresses the practicalities of characterizing this population: what a tremendous effort it would be to find the ages of *every one* of those cars! – whuber Oct 07 '13 at 13:53
  • @whuber Yes. Indeed it could be misunderstood in more ways than you mention. I should have phrased it better. – Glen_b Oct 07 '13 at 14:11
  • @whuber: In the UK at any rate it wouldn't be much effort - the Driver and Vehicle Licensing Authority holds make, model, & date of manufacture of all on-road cars in its database. They wouldn't tell you the answer for free of course. – Scortchi - Reinstate Monica Oct 07 '13 at 16:48
  • @Scortchi That's a nice example of a point I think is paramount here: the cost of obtaining the population statistics determines whether the company will do that or will instead estimate them from a sample. It also shows how important it is to define the population clearly: Most automobile manufacturers have their products in a large number of countries, not just the UK. – whuber Oct 07 '13 at 17:23

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