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Re: dummy coding of categorical variables results in zero coefficients a


From: EKreyken
Subject: Re: dummy coding of categorical variables results in zero coefficients and standard errors
Date: Wed, 20 Dec 2023 10:44:18 -0800

Hi Time,

Answering the stats stuff only:

Without seeing the data, I can’t make a definitive call on this, but it seems 
that calculations for the t test cannot be completed properly. This indicates 
either a) no shared linear variability in your XY variables (incredibly rare to 
get perfect 0), forcing the t test to do a division of 0 over the variability 
in X, or b) your X variable has no variability (also incredibly rare), forcing 
the t test to divide shared linear variability over 0. 

You may have one of several problems. 
1) dummy coding wasn’t done accurately and you don’t have enough variability in 
your X variable
2) you have a perfect non-relationship in those variables (very very rare)
3) you have a non-linear relationship that is really acting weird with the 
t-test, but it wouldn’t yield a n/a. 

Check the following: 
A) get the averages and standard deviations for all predictors. They should 
have a standard deviation above 0 for the test to work. 

B) plot the data for those specific predictors (X) against the response (Y) 
variable. You should end up with 2 separate graphs.  Check for linear /non 
linear patterns. 

C) plot the residuals (google/youtube knows how to do this). Check for 
“randomness” vs patterns: patterns in the residuals are bad. 

If you’re interested, I offer stats consulting. 

Cheers,
Elisabeth 

On Dec 20, 2023, at 1:47 AM, tim.goodspeed@btinternet.com wrote:

found?


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