Consumer Heterogeneity and Paid Search Effectiveness: A Large‐Scale Field Experiment

Internet advertising has been the fastest growing advertising channel in recent years, with paid search ads comprising the bulk of this revenue. We present results from a series of large‐scale field experiments done at eBay that were designed to measure the causal effectiveness of paid search ads. Because search clicks and purchase intent are correlated, we show that returns from paid search are a fraction of non‐experimental estimates. As an extreme case, we show that brand keyword ads have no measurable short‐term benefits. For non‐brand keywords, we find that new and infrequent users are positively influenced by ads but that more frequent users whose purchasing behavior is not influenced by ads account for most of the advertising expenses, resulting in average returns that are negative.     

Focal points in pure coordination games: An experimental investigation

This paper reports an experimental investigation of the hypothesis that in coordination games, players draw on shared concepts of salience to identify focal points on which they can coordinate. The experiment involves games in which equilibria can be distinguished from one another only in terms of the way strategies are labelled. The games are designed to test a number of specific hypotheses about the determinants of salience. These hypotheses are generally confirmed by the results of the experiment.     

Good Practice in the Education of Children in Residential Care

Correspondence to The Directors, New Horizons (Child Care) Ltd., Unit B, Silkmoor, New Street, Frankwell, Shrewsbury, Shropshire SY3 8LN, UK. E-mail info{at}new-horizonschildcare.co.uk Summary Research shows that the education of children in residentialcare is generally poor, in terms of both process and outcome.This has highlighted the uncertainty over how best to educatethese children and a pessimism over what they can achieve. Drawingupon the findings of an evaluation of a children’s home,this paper argues that children in residential care can havegood educational outcomes. It also shows that it is possibleto identify the processes by which these outcomes can be broughtabout. However, this largely positive assessment is predicatedupon education being defined broadly in respect of ‘process’and ‘outcome’. Moreover, it is important not tounderestimate the formidable challenges inherent in this work.While the multi-agency nature of this work gives rise to oneof the most significant, it has to be recognized that responsibilityfor the education of children in residential care falls primarilyto children’s homes. If children’s homes are tomeet the increasing expectations that are being made of them,national and local government will have to undertake a radicaltransformation of residential childcare, especially in termsof attitudes towards, and resources invested in, this sector.Central to this is the establishment of a qualified and properlysupported workforce.     

Decentring the NHS: A Case Study of Resource Allocation Decisions within a Health District

The aim of New Labour's health policy is to shift more of the balance of power and responsibility for services to the local level. But, while the government proclaims a new decentralized NHS, doubts exist about the extent to which the reality on the ground matches the tone of policy. This article reports empirical work examining the level of autonomy purchasers have over budgetary allocation. A case study analysis of purchasing within a single district was undertaken for the financial year 2001/2 which included semi‐structured interviews with key officers responsible for budget allocation. Purchasers approach a new financial year with a starting position that matches the previous year's allocation—the “baseline”, this is adjusted for inflation and, as has happened over the last few years, increased further in real terms by “growth funds” for service modernization and government initiatives. The analysis shows a clear dissonance between policy and practice; although purchasers have complete control over their “baseline budgets”, the study found that this does not “ring true” at the local level. Only about a fifth of growth funds were at the discretion of purchasers as most are taken by national priorities and pay and price inflation. Further decentralization is planned, which includes transferring more control of funds to primary care trusts by 2004, the extent to which these measures will change the perceptions of those working in the service remains to be seen—only then will the government be able to claim a truly decentred service.     

Testing for Additivity at Select Mixture Groups of Interest Based on Statistical Equivalence Testing Methods

Several assumptions, defined and undefined, are used in the toxicity assessment of chemical mixtures. In scientific practice mixture components in the low-dose region, particularly subthreshold doses, are often assumed to behave additively (i.e., zero interaction) based on heuristic arguments. This assumption has important implications in the practice of risk assessment, but has not been experimentally tested. We have developed methodology to test for additivity in the sense of Berenbaum (Advances in Cancer Research, 1981), based on the statistical equivalence testing literature where the null hypothesis of interaction is rejected for the alternative hypothesis of additivity when data support the claim. The implication of this approach is that conclusions of additivity are made with a false positive rate controlled by the experimenter. The claim of additivity is based on prespecified additivity margins, which are chosen using expert biological judgment such that small deviations from additivity, which are not considered to be biologically important, are not statistically significant. This approach is in contrast to the usual hypothesis-testing framework that assumes additivity in the null hypothesis and rejects when there is significant evidence of interaction. In this scenario, failure to reject may be due to lack of statistical power making the claim of additivity problematic. The proposed method is illustrated in a mixture of five organophosphorus pesticides that were experimentally evaluated alone and at relevant mixing ratios. Motor activity was assessed in adult male rats following acute exposure. Four low-dose mixture groups were evaluated. Evidence of additivity is found in three of the four low-dose mixture groups. The proposed method tests for additivity of the whole mixture and does not take into account subset interactions (e.g., synergistic, antagonistic) that may have occurred and cancelled each other out.     

Buehler confidence limits and nesting

The Buehler 1 –α upper confidence limit is as small as possible, subject to the constraints that its coverage probability is at least 1 –α and that it is a non‐decreasing function of a pre‐specified statistic T. This confidence limit has important biostatistical and reliability applications. Previous research has examined the way the choice of T affects the efficiency of the Buehler 1 –α upper confidence limit for a given value of α. This paper considers how T should be chosen when the Buehler limit is to be computed for a range of values of α. If T is allowed to depend on α then the Buehler limit is not necessarily a non‐increasing function of α, i.e. the limit is ‘non‐nesting’. Furthermore, non‐nesting occurs in standard and practical examples. Therefore, if the limit is to be computed for a range [α_L, α_U]of values of α, this paper suggests that T should be a carefully chosen approximate 1 –α_L upper limit for θ. The choice leads to Buehler limits that have high statistical efficiency and are nesting.