Hi, I'm looking at creating some aggregrate table based on ~2hr pull from the main DB and looking to get standard stuffs like min/max/ave etc.
I'm having some issues with getting stdev which would be representative of the stdev of say 10 hours of data. >From this website : (it references using SQL Server Analysis services but I think the concept is the same for MySQL) 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. All I know is that, it works. Only question is, why? Can anyone explain this? -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/[EMAIL PROTECTED]
