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st: Getting Negative Alphas


From   "Chao Yawo" <Yawo1964@yahoo.com>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: Getting Negative Alphas
Date   Sun, 16 Nov 2008 11:19:47 -0500

Hi,

I am trying to create a summed scale of 3 items which are coded as follows:

Respondents were asked whether they

1. Have had an STD in the last 12 months: coded No (0)  and 1 (Yes)
2. Used condom during last intercourse: coded No (0) and 1 (Yes)
3  How many non-marital partners they've had sexual relationship with:
range from 0 to 20

I want the summed scale to reflect the degree of risky sexual
behavior, so i followed these steps in recoding the variables:

1. STD experience:  the original coding is ok, since the higher
numerical code (1 -  that an individual has had an STD) is consistent
with the meaning intended.
2. Condom use: since the question wording is inconsistent with the
coding - ie, a code of 1 implies that the individual did use condom
and hence not engaged in risky behavior, I reverse coded the variable
so that the original 0s are 1, and the original 1s now take 0.
3. Non-marital sexual experience: since the original coding indicated
the actual number of non-marital partners, I merely recoded the
variable to reflect those who did not have any extra-marital
relationships (a code of 0), and those who had any (a code of 1). Here
again, having a non-marital experience is reflective of a risky sexual
behavior.

Now, since the individual items are dichotomous, I had to use the
Kuder-Richardson coefficient of reliability (see:
http://ideas.repec.org/c/boc/bocode/s351001.html), instead of the
regular Cronbach's alphas which is intended for multi-point items.

Stata keeps returning a negative alpha of   -0.72 for the 3 items. I
am wondering why this is the case since I believe I have correctly
recoded all the variables to ensure that higher numerical codes give
consistent meanings.  I am therefore stumped wondering what I might be
doing wrong.

I would appreciate any pointers to correct this anomaly.

Thanks very much,

CY
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