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2005-05-15

I am wondering if there might be someone in this platform that might be kind enough to let me know how to translate a square raw data matrix to ANY sort of distance (diagonal) matrix suitable for input to PROC MDS in SAS ?

[此贴子已经被作者于2005-5-25 11:12:33编辑过]

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2005-5-24 05:18:00
Question 1: Why The MDS and ALSCAL procedures in SAS may sometimes produce different results?
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2005-5-25 01:41:00

The MDS and ALSCAL procedures may sometimes produce different results for the following reasons:

  • With the LEVEL=INTERVAL option, PROC MDS fits a regression model while PROC ALSCAL fits a measurement model. These models are not equivalent if there is more than one partition, although the differences in the parameter estimates are usually minor.
  • PROC MDS and PROC ALSCAL use different algorithms for initialization and optimization. Hence, different local optima may be found by PROC MDS and PROC ALSCAL for some data sets with poor fit. Using the INAV=SSCP option causes the initial estimates from PROC MDS to be more like those from PROC ALSCAL.
  • The default convergence criteria in PROC MDS are more strict than those in PROC ALSCAL. The convergence measure in PROC ALSCAL may cause PROC ALSCAL to stop iterating because progress is slow rather than because a local optimum has been reached. Even if you run PROC ALSCAL with a very small convergence criterion and a very large iteration limit, PROC ALSCAL may never achieve the same degree of precision as PROC MDS. For most applications, this problem is of no practical consequence since two- or three-digit precision is sufficient. If the model does not fit well, obtaining higher precision may require hundreds of iterations.
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2005-5-25 09:36:00

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2005-5-25 11:18:00
SAS Proc Autoreg takes care of the serial correlation in the error term. PA computes predicted values based on the structural model and the full model. The structural piece uses only the parameter estimates of the independent variables (no error terms). The full model adds back the AR1 error term to the model. I'm having trouble reconciling the computation of the 1st predicted value for the full model. SAS uses a Kalman filter to do this, and the calculation appears complex. Can anyone explain how the calculation is done for a simple (y regressed on time w/AR1 error term) regression, and refer me to a good reference source.
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2005-5-25 11:31:00

There are a lot of references for Kalman filters. The best source certainly are

1. Peter S. Maybeck. Stochastic Models, Estimation, and Control, volume 1. Academic Press, New York, 1979.

2. Applied Optimal Estimation by Arthur Gelb

3. A.H. Jazwinski, Stochastic Processes and Filtering Theory, Academic Press, New York, 1970

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