Influences on recruitment to randomised controlled trials in mental health settings in England: a national cross-sectional survey of researchers working for the Mental Health Research Network

Background: Recruitment to trials is complex and often protracted; selection bias may compromise generalisability. In the mental health field (as elsewhere), diverse factors have been described as hindering researcher access to potential participants and various strategies have been proposed to overcome barriers. However, the extent to which various influences identified in the literature are operational across mental health settings in England has not been systematically examined. Methods: A cross-sectional, online survey of clinical studies officers employed by the Mental Health Research Network in England to recruit to trials from National Health Service mental health services. The bespoke questionnaire invited participants to report exposure to specified influences on recruitment, the perceived impact of these on access to potential participants, and to describe additional positive or negative influences on recruitment. Analysis employed descriptive statistics, the framework approach and triangulation of data. Results: Questionnaires were returned by 98 (58%) of 170 clinical studies officers who reported diverse experience. Data demonstrated a disjunction between policy and practice. While the particulars of trial design and various marketing and dommunication strategies could influence recruitment, consensus was that the culture of NHS mental health services is not donducive to research. Since financial rewards for recruitment paid to Trusts and feedback about studies seldom reaching frontline services, clinicians were described as distanced from research. Facing continual service change and demanding clinical workloads, clinicians generally did not prioritise recruitment activities. Incentives to trial participants had variable impact on access but recruitment could be enhanced by engagement of senior investigators and integrating referral with routine practice. Comprehensive, robust feasibility studies and reciprocity between researchers and clinicians were considered crucial to successful recruitment. Conclusions: In the mental health context, researcher access to potential trial participants is multiply influenced. Gatekeeping clinicians are faced with competing priorities and resources constrain research activity. It seems that environmental adjustment predicated on equitable resource allocation is needed if clinicians in NHS mental health services are to fully support the conduct of randomised controlled trials. Whilst cultural transformation, requiring changes in assumptions and values, is complex, our findings suggest that attention to practical matters can support this and highlight issues requiring careful consideration

Analysis and Design of Robust Key Schemes for Multicast Communications

Recent literature presents several rooted tree based member deletion/revocation schemes trying to simultaneously minimize the key storage whileproviding efficient member deletion/revocation. Many of these approaches have different solutions and provide different values for the number of keys to be stored and distributed. In this paper, we show that these problems can be systematically studied using basic concepts from information theory. In particular, we show that the entropy of member revocation event plays a major role in defining the key allocation requirements. We then relate the entropy of member revocation event to bounds on the key length. We also show that the optimal Huffman coding strategy used in leads to security weaknesses. A method for generating key management schemes to withstand varying degrees of member collusion is also presented.ATIRP 4th Annual Conference</i

On a Recent Problem of Communication-Storage Tradeoffs for Secure Multicast for Large Networks

A variety of rooted tree-based secure multicast networks with different efficiencies and storage requirements that are linear in group size have been proposed. Recently, Canetti et. al. presented a scheme based on clustering that had sub-linear storage requirements at the group controller. However, they were unable to prove or disprove that the scheme was optimal, and posed it as an open question. In this paper we answer the question with affirmative NO! Additionally we use our results to show that the optimal clustering in this context is related to maximum entropy of member revocation event, and corresponding optimal strategy is to partition members such that each cluster has the same probability of being revoked.Second Workshop on Security in Communication Networks</i

Optimal Scalable Security Architectures in the Presence of Colluding Mobile Traitors

We present the issues related to secure muiticast communication in the presence of members who may collaborate to compromise the integrity of the system's security. We also show that the ability to compartmentalize the system compromise depends on the availability of trusted intermediate nodes. We also note that some variations of the recently proposed tree-based schemes don't provide the required level of security and may be compromised if two appropriate members are compromised. We present the analysis of the weakness of these schemes here. We further note that the currently available tree-based key distribution schemes are not optimal, and choose the worst case solution for key assignment. We note that the claims, including the collusion, can be formally proved using basic concepts from source coding theory and entropy.1999 IEEE Emerging Technologies Symposium on Wireless Communications and Systems</i

Security Analysis of a Distributed Common Secret Generation Procedure

In a distributed scheme allowing any number of members to compute a commonsecret without revealing individual secret was proposed. We present asecurity weakness of this protocol. In doing so, we show that any twomembers can collude and obtain the secret contributed by middle memberin generating the common secret.Journal of Cryptology</i

An Information Theoretic Approach for Design and Analysis of Rooted-Tree Based Multicast Key Management Schemes

Recent literature presents several rooted tree based member deletion/ revocation schemes trying to simultaneously minimize the key storage while providing efficient member deletion/revocation. Many of these approaches have different solutions and provide different values for the number of keys to be stored and distributed. In this paper, we show that many of these papers can be systematically studied using basic concepts from information theory. In particular, we show that the entropy of member revocation event plays a major role in defining the key allocation requirements. We then relate the entropy of member revocation event to provide bounds on the key length. We also show that the optimal Huffman coding strategy used leads to security weakness. A method for generating key management schemes with varying degrees of member collusion is also presented in this paper.Journal of IEEE Transaction on Information Theory</i

A Decision-Process Analysis of Implicit Coscheduling

This paper presents a theoretical framework based on Bayesian decision theory for analyzing recently reported results on implicit coscheduling of parallel applications on clusters of workstations. Using probabilistic modeling, we show that the approach presented can be applied for processes with arbitrary communication mixes. We also note that our approach can be used for deciding the additional spin times in the case of spin-yield.Finally, we present arguments for the use of a different notion of fairness than assumed by prior work.International Conference on Parallel and Distributed Computing</i

Dynamic Elgamal Public Key Generation with Tight Binding

We present a new distributed, group ElGamal public key generation algorithm which also permits maintenance of a group-specific, dynamic, individual ElGamal public key infrastructure. We parameterize the group with a time-varying quantity that serves as a distributed mechanism for controlling key generation privilege. Our scheme can be viewed as an alternative to polynomial schemes where, at the time of the secret construction step, there has to be a third party or a black box to combine the shares. Also, in polynomial schemes, at the time of combining,the individual shares of the secret have to be revealed to the third party. In our scheme, the common secret can be generated without ever exposing the individual shares constructing it. We note that many of the recently proposed distributed key management schemes need such group keys for certification and signing purposes.3rd Annual Conference on Advanced Telecommunications and Information Distribution Research Program (ATIRP)</i

A Distributed Shared Key Generation Procedure Using Fractional Keys

We present a new class of distributed key generation and recoveryalgorithms suitable for group communication systems where the groupmembership is either static or slowly time-varying, and must be tightlycontrolled. The proposed key generation approach allows entities whichmayhave only partial trust in each other to jointly generate a shared keywithout the aid of an external third party. The group collectivelygenerates and maintains a dynamic group parameter, and the shared key isgenerated using a strong, one-way function of this parameter. This schemealso provides perfect forward secrecy. The validity of key generation canbe checked using verifiable secret sharing techniques. The key retrievalmethod does not require the keys to be stored in an external retrievalcenter. We note that many Internet-based applications may have theserequirements. Fulfillment of these requirements is realized through theuse of fractional keys--a distributed technique recently developed toenhance the security of distributed systems in a non-cryptographicmanner