I've worked with some data like these and have observed similar patterns. Measures of internal consistency reliability assume that the observed indicators reflect are generated by a (or perhaps more than one) an underlying latent variable (e.g., scores on observed indicators of depression are produced by the degree to which subjects are depressed, plus measurement error). I'm not sure that's reasonable with these indicators. One might argue that having an STD might be caused by behaviors such as number of unmarried sexual partners and condom use. And one might expect that individuals who have sex with numerous unmarried partners are more likely to use condoms. In some of our data we've observed that frequency of condom use is positively associated with other indicators of sex risk and some of the descriptions of the Risk Assessment Behavior sex risk composite have reported low reliabilities and have argued that this occurs because the indicators are better conceptualize!
d as causes of sex-risk rather than reflections of sex-risk. In short, measures of internal-consistency reliability imply a measurement model that is probably untenable in this case.
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Chao Yawo
Sent: Sunday, November 16, 2008 11:20 AM
To: [email protected]
Subject: st: Getting Negative Alphas
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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