How to assess?
Message posted by Ruby Lo (via 203.193.106.164) on November 17, 2001 at 12:54 AM (ET)

Hello editor,
My name is Ruby from Hong Kong, I have a problem that I have got a set of data - e.g. the unemployment figues of each month from 1/2000 to 10/2001. My boss told me to divide them in a plot graph with three lines. (1) The mean, (2) A line above the mean and (3) A line below the mean. The purpose is to categorize the figues in three section:(1) The figures that over the upperest line is red that means it is high risk - the Government should be alerted and aware, (2) the middle section is amble - normal situation, (3) The section below the lowerest line is green - the least risk level. I tried to calcalate the mean, the (+)(-)standard deviation to divide them into three sections, however, it seems not right since the upperest line always higher than the biggest number of the set data. Thus, it never go to the red zone. I tried other sets of data but the result is the same. My boss says why all the figures always at the amble zone only?
Can you help me how to assess the data and how to set up the range? Thank you very much.
Best wishes,
Ruby

READERS RESPOND:
(In chronological order. Most recent at the bottom.)

Re: How to assess?
Message posted by Tomi (via 154.32.143.180) on November 18, 2001 at 1:16 AM (ET)

You may want to look at http://www.statsoftinc.com/textbook/stquacon.html for more detailed information on control charts, but here is a quick fix for you:

Use 1.96 * standard deviation / sqrt(n) where n is the number of observations.

Re: How to assess?
Message posted by Darius (via 200.23.217.10) on November 19, 2001 at 10:22 AM (ET)

Total Variation = Variation within Subgroup + Variation between subgroup

Its wrong to use std/sqrt(n) because of the estimate of variation you where using.

It looks like individual control charts so http://www.qualitymag.com/articles/jan98/0198wh.html could help you.

There are other interesting articles and sites:

http://www.qualitymag.com/articles/1998/feb98/0298wh.html

http://www.qualitymag.com/articles/feb99/0299wh.html

http://www.qualitymag.com/articles/1999/aug99/0899wh.html

http://www.qualitymag.com/articles/1999/sep99/0999wh.html

http://www.qualitymag.com/articles/1999/oct99/1099wh.html

http://www.qualitymag.com/articles/oct98/1098wh.html

look for "individual control charts" in

http://www.itl.nist.gov/div898/handbook/

I hope, this may help you.