Statisticians_group : Message: Re: [Statisticians_group] Problem related to time series analysis

hi Swathi,

If you use different degrees of polynomials are used for fitting trend line, then you should use adjusted R square which is not displayed in Excel. So, you use any stat. software that gives adjusted R-square

Hope this will help you.
S.Ravindran

----- Original Message ----
From: swathi shetty <swathisona_shetty@...>
To: Statisticians_group@...
Sent: Tuesday, October 10, 2006 10:56:41 PM
Subject: [Statisticians_group] Problem related to time series analysis

Hi all,

Can anybody help me in solving the below mentioned problems

Excel has a number of tools to assist in analyzing trends in data. Excel offers six choices of types of trend lines that might be applied to the charted data, namely

Linear
Logarithmic
Polynomial
Power
Exponential
Moving Average

And along with these trend line, if necessary excel will also display the R² value and the equation.

My problem here is in interpreting the R²value, based on the R² value we select one of the trend line which best fits the data and use it to predict the future values, but the problem in case of polynomial regression is when we increase the degree of the polynomial the R² value increases. What decision has to be taken in this case? Do we have to select the model with highest degree, since it has the highest R² value?

In case of time series analysis with one set of study variable, when we fit a trend line using polynomial regression

Y=b+c₁X+c₂X²+------------+c₆X⁶in excel the values for X are taken as 1, 2, 3, 4, 5, --------------, as the number of observation increases the X value also increases and therefore the future predicted response values(Y) will be too big for the model with positive coefficient which is far from reality. How to over come this problem?

Looking forward for the solutions.

Kind Regards

Swathi.

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Hi all, Can anybody help me in solving the below mentioned problems Excel has a number of tools to assist in analyzing trends in data. Excel offers six choices...

swathi shetty
swathisona_s...

Oct 11, 2006
7:05 am

Swathy, 1. U must know or hypothesie the nature of data and accordingly, in principle, confirm such nature by use of fitting function. 2. R-sqr will increase...

surendra barsode
sbbarsode

Oct 11, 2006
9:21 am

Excel is a very bad environment in which to do statistics for several reasons. See: http://www.burns-stat.com/pages/Tutor/spreadsheet_addiction.html That...

Patrick Burns
burnsstat

Oct 11, 2006
10:20 am

... Spreadsheets have many advantages: 1. Accessible user interface. 2. Easily table and chart making. 3. Wide knowledge base among the technical population. ...

Robert A LaBudde
rlabudde

Oct 12, 2006
3:36 am

hi Swathi, If you use different degrees of polynomials are used for fitting trend line, then you should use adjusted R square which is not displayed in Excel....

S.Ravindran
sravinthiran

Oct 12, 2006
3:26 am

... Excel reports Adjusted R^2 for regression fits. ================================================================ Robert A. LaBudde, PhD, PAS, Dpl. ACAFS...

Robert A LaBudde
rlabudde

Oct 12, 2006
8:02 am

... I. Although R^2 (or better, the "adjusted" R^2) is frequently recommended for stepwise fit assessment, this measure is flawed, as are all correlation-based...

Robert A LaBudde
rlabudde

Oct 12, 2006
3:27 am

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