Re: st: 1-4 scale

[Richard Williams <richardwilliams.ndu@gmail.com>]

At 10:57 AM 8/5/2012, David Hoaglin wrote:
> Dear Ebru,
>
> People often analyze data from Likert scales as equally spaced, so you
> can use each of the eight items in your model as a numerical variable,
> with values 1 to 4.  You simply need to be aware that you are treating
> the four categories as equally spaced.

It is a debatable practice though. Consider the following (warm has 4 values):

use "http://www.indiana.edu/~jslsoc/stata/spex_data/ordwarm2.dta";, clear
reg warm yr89 male white age ed prst
rvfplot

According to the reference manual discussion of rvfplot, "In a 
well-fitted model, there should be no pattern to the residuals 
plotted against the fitted
values...Any pattern whatsoever indicates a violation of the 
least-squares assumptions."

Clearly, there is a pattern in the above rvfplot, i.e. you get 4 
parallel straight lines. Further, it isn't unique to this example; 
any 4 category dependent variable will show the same thing.

In fairness, if your dependent variable had 17 possible values, you 
would have 17 straight lines -- but your eye probably wouldn't detect 
that because everything would seem so cluttered. There is probably 
some point where there are enough possible values that violations of 
OLS assumptions aren't important, but I would be hesitant to say that 
point is met with a DV that only has 4 categories.

> Earlier you asked about centering those variables.  Centering will do
> no harm.  As far as the model is concerned, it affects only the
> definition of the intercept.  If you do decide to "center" the
> variables, you may want to use one of the four values.  If the data on
> an item are not concentrated at one end, you could use 2 or 3 or
> perhaps 2.5 as the centering constant.  (In a 5-point Likert scale
> with a neutral category at 3, using 3 would often be a reasonable
> choice.)
>
> When you have the results from the model with the eight separate
> items, you may want to see whether the coefficients for the four items
> within a heading are similar.  If they are, and it makes sense, you
> could consider replacing those four items with their sum (or average)
> --- a composite score.
>
> David Hoaglin
>
> On Sun, Aug 5, 2012 at 2:35 AM, Ebru Ozturk <ebru_0512@hotmail.com> wrote:
> > Dear David,
> >
> > I use tobit estimation. If I enter each item (8) in the question 
> (objectives of innovation) into the model without changing their 
> values, leaving them as they are (1 to 4), does it work? Because 
> sometimes tobit can create problems if it has many variables in the model.
>
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Richard Williams, Notre Dame Dept of Sociology
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