A comparison of reliability coefficients for psychometric tests that consist of two parts

If a test consists of two parts the Spearman-Brown formula and Flanagan's coefficient (Cronbach's alpha) are standard tools for estimating the reliability. However, the coefficients may be inappropriate if their associated measurement models fail to hold. We study the robustness of reliability estimation in the two-part case to coefficient misspecification. We compare five reliability coefficients and study various conditions on the standard deviations and lengths of the parts. Various conditional upper bounds of the differences between the coefficients are derived. It is shown that the difference between the Spearman-Brown formula and Horst's formula is negligible in many cases. We conclude that all five reliability coefficients can be used if there are only small or moderate differences between the standard deviations and the lengths of the parts

Kappa coefficients for dichotomous-nominal classifications

Two types of nominal classifications are distinguished, namely regular nominal classifications and dichotomous-nominal classifications. The first type does not include an 'absence' category (for example, no disorder), whereas the second type does include an 'absence' category. Cohen's unweighted kappa can be used to quantify agreement between two regular nominal classifications with the same categories, but there are no coefficients for assessing agreement between two dichotomous-nominal classifications. Kappa coefficients for dichotomous-nominal classifications with identical categories are defined. All coefficients proposed belong to a one-parameter family. It is studied how the coefficients for dichotomous-nominal classifications are related and if the values of the coefficients depend on the number of categories. It turns out that the values of the new kappa coefficients can be strictly ordered in precisely two ways. The orderings suggest that the new coefficients are measuring the same thing, but to a different extent. If one accepts the use of magnitude guidelines, it is recommended to use stricter criteria for the new coefficients that tend to produce higher values

Additive kappa can be increased by combining adjacent categories

Weighted kappa is a measure that is commonly used for quantifying similarity between two ordinal variables with identical categories. Additive kappa is a special case of weighted kappa that allows the researcher to specify distances between adjacent categories. It is shown that additive kappa is a weighted average of the additive kappas of all collapsed tables of a specific size. It follows that, if the reliability of a categorical rating instrument is assessed with additive kappa, the reliability can be increased by combining categories.<br/

A Peer Outreach Initiative to Increase the Registration of Minorities as Organ Donors

Background Black, Asian and minority ethnic (BAME) communities are disproportionately affected by inequalities in transplant services in the UK. There are some indications from pilot programmes that appeals for BAME organ donors may be more effectively communicated by employing grassroots, community-networking approaches, but such initiatives have not been adequately described or evaluated. Methods Lay individuals from BAME communities were trained as peer outreach workers. They attended a series of public events to promote knowledge of organ donation and transplantation among the public. Information was gathered from 806 evaluation forms completed by event attendees at 34 separate events. From these, 54 follow-up interviews were conducted with event attendees who completed evaluation forms, indicated that they intended to sign up to the NHS Organ Donor Register (ODR) within the next month and consented to follow-up. Results Peer outreach initiatives of the type evaluated are associated with increased numbers of BAME people registering as organ donors. A total of 8.8% of event attendees signed up to the NHS ODR. The programme was most effective with people who had previously considered becoming organ donors but who did not know how to go about it. It was less effective with people who had not previously considered it, or who were scared about signing up, or who feared family or religious disapproval. Conclusions Peer outreach programmes with BAME communities can be an effective way of reducing inequalities by increasing the number of people on the NHS ODR and encouraging people to think about the issue

Ordering Properties of the First Eigenvector of Certain Similarity Matrices

It is shown for coefficient matrices of Russell-Rao coefficients and two asymmetric Dice coefficients that ordinal information on a latent variable model can be obtained from the eigenvector corresponding to the largest eigenvalue

Properties of Bangdiwala's B

Cohen's kappa is the most widely used coefficient for assessing interobserver agreement on a nominal scale. An alternative coefficient for quantifying agreement between two observers is Bangdiwala's B. To provide a proper interpretation of an agreement coefficient one must first understand its meaning. Properties of the kappa coefficient have been extensively studied and are well documented. Properties of coefficient B have been studied, but not extensively. In this paper, various new properties of B are presented. Category B-coefficients are defined that are the basic building blocks of B. It is studied how coefficient B, Cohen's kappa, the observed agreement and associated category coefficients may be related. It turns out that the relationships between the coefficients are quite different for 2x2 tables than for agreement tables with three or more categories