(i) Let {x i : i 5 l, 2, ..., n} be a set of n data...

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(i) Let {xi: i 5 l, 2, ..., n} be a set of n data points, and let x̅ be the average. Suppose that the units i are divided into two groups of sizes nl and n2, with nl + n2 = n. Without loss of generality, order the observations as

nl'nl+l'

O that the data points for the first group appear first. Let

be the averages for the two groups. Show that

so that x̅ can be expressed as a weighted average of the averages from the two subgroups.

(ii) Do the weights w1 and w2 in part (i) make intuitive sense? Explain.

(iii) How does the finding in part (i) extend the case of g groups, where the group sizes are

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