Just wondering how everyone is doing aggregration of production data. Data which runs in the vicinity of a few million a week. What are the methods which will effectively provide the min/max/average/count/stdev of the weekly sample size based on different products/software mix etc. and still be able to answer correctly, what's the average of data_1 over the pass 2 months? I can't just take the average of an 8 averages of each week) eg: wk avg data_1 w1 - 2 w2 - 2 w3 - 2 w4 - 3 w5 - 1 w6 - 2 w7 - 2 w8 - 2 average of past 2 months = ave(w1-w8) which is statistically wrong. using sum of data_1 per week would work though. Please share your expertise / experience. (getting min/max/count isn't much of an issue. Stdev is the main issue I believe) One such instance I've read about is.. >From this website : (it references using SQL Server Analysis services but I think the concept is the same) http://www.phptr.com/articles/printerfriendly.asp?p=337135&rl=1 1. Calculate sum of square of each sale 2. multiple the result of step 1 by the sales count 3. sum all sales 4. Square the result of step 3 5. Substract the result of step 4 from the result of step 2 6. Multiply the sales count by one less than sales count ("sales_count" * ("sales_count" - 1)) 7. Divide the result of step 5 by the result of step 6 8. Stdev will be the square root of step 7 The results are valid (verified with actual data) but I don't understand the logic. All the Statistical books I've read marked stdev as sqrt (sum(x - ave(x))^2 / (n - 1). The formula is very different, hence the confusion. ---------------------------(end of broadcast)--------------------------- TIP 4: Have you searched our list archives? http://archives.postgresql.org/
