The Elsmar Cove Forum Error Accumulation - Combined Result without going to Partial Derivitaves
 Forum User Name Keep Me Logged In Password
 Register Photo Albums Blogs FAQ Registered Visitors Social Groups Calendar Search Today's Posts Mark Forums Read

 Elsmar Cove Forum Visitor Notice(s) It is the Labor Day holiday weekend in the US so activity in the forum will be slow for a few days. Normal activity in the forum will start to pick up again mid-week. Please see the forum Calendar for more information on the holiday.Logged In Registered Members can click the red X to close this notice box.

 Search the Elsmar Cove @import url(http://www.google.com/cse/api/branding.css); Custom Search Monitor the Elsmar Forum Monitor New Forum Posts Follow Marc & Elsmar Elsmar Cove Groups Sponsor Links Donate and \$ Contributor Forum Access Courtesy Quick Links Links that Elsmar Cove visitors will find useful in your quest for knowledge: Howard'sInternational Quality Services Marcelo Antunes'SQR Consulting Bob Doering'sCorrect SPC - Precision Machining NIST's Engineering Statistics Handbook IRCA - International Register of Certified Auditors SAE - Society of Automotive Engineers Quality Digest Portal IEST - Institute of Environmental Sciences and Technology ASQ - American Society for Quality

#1
24th May 2010, 01:27 AM
 Statisticale Getting Involved (6 to 9 Posts)   Registration Date: Dec 2009 Posts: 8 Thanks Given to Others: 1 Thanked 0 Times in 0 Posts Karma Power: 20 Karma: 10
Error Accumulation - Combined Result without going to Partial Derivitaves

Hi,

I have an area of forest that has been stratified into 2, & I wish to calculate the +/- 3 standard deviation (sigma) distribution of tree heights in the combined area. The weighting of the first area is 0.73, & the second is 0.27. Means heights are 8.3 & 5.5 meters, with standard deviations of 1.2 & 0.9 meters.

If we make the assumption that co-variance can be ignored, can the combined result be calculated without going to partial derivitaves? If so, how?

Best regards.

#2
24th May 2010, 09:24 AM
 Darius Appreciated Information Resource   Registration Date: Mar 2002 Location: Monterrey Mexico Age: 52 Posts: 530 Thanks Given to Others: 69 Thanked 224 Times in 148 Posts Karma Power: 111 Karma: 3551
Re: Error Accumulation - Combined Result without going to Partial Derivitaves

Sorry to ask for more info..., what are you searching?. That you can calculate something doesn't mean you should.

If you need to inform a data value (standard deviation for all set), maybe could be better to obtain the weighted mean for stdev or much better could be to calculate the correlation between height and variation and don't show a value but it's equation.

IMHO, if you want to do SPC with such data, you can transform the data to Z values (a Zed chart), so the limits will be between 3 and -3 sigma, and you can chart both kind of products (tree-heights) as one set of data.

__________________

Vits er thorf, theim vitha ratar; daelt er heima hvat; at augabragthi verthr, sá er ekki kann ok med snotrom sitr.
#3
27th May 2010, 11:05 AM
 Statistical Steven Statistician   Registration Date: May 2005 Location: New Jersey Age: 49 Posts: 974 Thanks Given to Others: 240 Thanked 362 Times in 252 Posts Karma Power: 143 Karma: 4239
Re: Error Accumulation - Combined Result without going to Partial Derivitaves

Quote:
 In Reply to Parent Post by Statisticale Hi, I have an area of forest that has been stratified into 2, & I wish to calculate the +/- 3 standard deviation (sigma) distribution of tree heights in the combined area. The weighting of the first area is 0.73, & the second is 0.27. Means heights are 8.3 & 5.5 meters, with standard deviations of 1.2 & 0.9 meters. If we make the assumption that co-variance can be ignored, can the combined result be calculated without going to partial derivitaves? If so, how? Best regards.
If you assume independence, you can use the property of variances to solve the problem.

Var (X + Y) = Var(X) + Vary (Y)
Var (0.73X) = (0.73^2)Var(X)
Var (0.27Y) = (0.27^2)Var(Y)
Var (0.73X + 0.27Y) = (0.73^2)Var(X) + (0.27^2)Var(Y)

The calculation of the means is straightforward.

Overall Mean = ((n1*0.73*mean1) + (n2*0.27*mean2))/(n1+n2)

Does that make sense. Of course the assumnption of independence is CRITICAL here.

__________________

Steven Walfish

 The Elsmar Cove Forum Error Accumulation - Combined Result without going to Partial Derivitaves

 Bookmarks

 Visitors Currently Viewing this Thread: 1 (0 Registered Visitors (Members) and 1 Unregistered Guest Visitors)

 Forum Posting Settings You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules

 Similar Discussion Threads Discussion Thread Title Thread Starter Forum Replies Last Post or Poll Vote apokalypsis Using Minitab Software 3 7th December 2011 07:12 PM Mike S. Nonconformance and Corrective Action 128 30th June 2011 03:56 PM Michael Malis US Medical Devices (21 CFR part 820) 23 31st March 2010 01:54 AM iwotic Using Minitab Software 3 27th October 2009 11:28 PM just67horns Problem Solving, Root Cause Fault and Failure Analysis 27 5th October 2006 02:42 PM

The time now is 07:12 AM. All times are GMT -4.