> jabs <jabenavi...@gmail.com> wrote
>> Hello folks
>> I am a physician who works in Mexico. I would like to predict the
>> weight of a fetus before birth through ultrasound measurements. There
>> are many studies which have published an equation or a formulae in
>> order to estimate fetal weight, and the equation has been obtained
>>from independent variables (parameters of ultrasound). Unfortunately,
>> none of  these studies has been done in Mexican population.
>> I have collected the birth weight (dependent variable) of almost 500
>> newborns (NB). I hav also collected 13 ultrasound measurements
>> (independent variables) per fetus in the 48 hours prior to birth
>> (prenatal stage). My goal is to find an equation or formula to predict
>> the weight of the baby using ultrasound variables (independent
>> variables). I have read about this and I think I have to run a linear
>> regression in which the dependent variable would be the birth weight,
>> and ultrasound variables would be included as independent variables.

> So far so good ....

>> According to what I have read, I have to choose a selection of
>> variables backwards method  by which I will obtain a linear model.

> Not good at all.  Backwards methods (and other automatic variable selection methods)
> are not good.  They are commonly used, but they are wrong.

> The
>> problem is that I have no experience on how to perform this. Even
>> though, I have tried to do it using SPSS software and after running
>> the regression, at the results window I get a series of data such us
>> tables (descriptive statistics, correlation, included/deleted
>> variables, a summary model, ANOVA, analysis of colinearity, excluded
>> variables), and Graphics. What is the right way to run the multiple
>> regression? How can I get the model from these data? Which data must
>> be included in the equation? Thanks in advance for your help.

> You might try asking on an SPSS list, for details of how to do things in SPSS,
> but which variables you should use is not dependent on software.  If you
> are trying to replicate previous results, you should use the same variables.

> With 500 newborns, you could use all 13 variables - unless there are collinearity problems.

> Or you might want to use something like principal component regression, or partial least squares;
> you might be concerned with possible nonlinear effects; there are other possibilities as well.

> Peter

> Peter L. Flom, PhD
> Statistical Consultant
> www DOT peterflomconsulting DOT com