tutorial_7 - 2012/13Semester2 Tutorial7 ST3239 1 The...

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2012/13 Semester 2 Tutorial 7 ST3239 1. “The grouping of population elements is normally based on a known auxiliary variable or attribute, but group is possible also when only a list of the population elements is available. For example, the first group could consist of the first 10% of the elements listed, the second group of the next 20%, and so on. This means it is always possible to take a proportional stratified sample. Since such a sample is always better than a simple random sample of the same size, we need never consider taking a simple random sample.” Comment. 2. Suppose that a city has 90,000 dwelling units, of which 35,000 are hourses, 45,000 are apartments, and 10,000 are condominiums. (a) You believe that the mean electricity usage is about twice as much for houses as for apartments or condominiums, and that the standard deviation is proportional to the man so that S 1 = 2 S 2 = 2 S 3 . How would you allocate a stratified sample of 900 observations if you wanted to estimate the mean electricity consumption for all households in the city?
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