Lifetime prediction and confidence bounds in accelerated degradation testing for lognormal response distributions with an Arrhenius rate relationship

Determining the lifetime of a product is an important component of quality assurance. Traditional life testing methods are infeasible for products that have been designed to have a very long lifetime because they require a lengthy testing period. An alternative method is accelerated degradation testing, where a response variable determining the usability of the product is measured over time under multiple accelerating stress levels. The resulting data are then used to predict the life distribution of the product under the design stress level. In this dissertation, several methods are proposed and studied for obtaining prediction bounds for the lifetime of a future product and confidence bounds for the mean lifetime of a product using accelerated degradation testing. The proposed model assumes that products are subjected to a constant accelerating stress. The response variable is measured once for each product, and failure occurs when the response variable crosses a predefined threshold. The model assumes the natural logarithm of the response variable has a normal distribution with a mean that follows an Arrhenius rate relationship and a standard deviation whose natural logarithm follows a quadratic function of the time. Three methods are presented for obtaining prediction bounds for the lifetime of a future product at the design stress level. These methods use the maximum likelihood, model-based bootstrap, and maximum likelihood predictive density approaches. Two methods are presented for obtaining confidence bounds for the mean lifetime of a product at the design stress level. These techniques represent the delta method and three different variations of the model-based nonparametric bootstrap approach. The performance of the various methods for obtaining lifetime prediction and confidence bounds are studied using a Monte Carlo simulation study. The results identify several promising approaches --Abstract, page iii

Priming and the Post-Prime Pause in Mixed Fixed-Ratio Schedules

The present study was designed to investigate the effects of a stimulus change inserted in the large component of a mixed fixed-ratio 10 fixed-ratio 100 schedule. In mixed fixed-ratio schedules, a run of responses which approximates the response requirement of the smaller fixed ratio occurs at the beginning of the larger fixed ratio. This run of responses and the pause it precedes is called a prime. In Experiment I, priming acquisition was compared in a mixed schedule in which a change in key-color followed completion of the first 10 responses of the fixed-ratio 100 component and a mixed schedule with no stimulus change. Primes were acquired more rapidly in the mixed schedule with the stimulus change and the number of responses in a priming run was less variable than occurred without the stimulus change. In Experiment II, the effect of the stimulus change on primes was further investigated by removal of the smaller fixed-ratio component or by varying the location of the stimulus change. Primes occurred only when the smaller fixed-ratio component was present. Varying the location of the stimulus change resulted in the transfer of control from the external stimulus to response-produced stimuli. Future investigation of this point of transfer should prove useful in the study of the proprioceptive stimulus control of homogeneous behavior sequences. In Experiments Ill through VI, the variables controlling the length of the post-prime and the post-reinforcement pauses were investigated using the mixed FR x chained FR x FR y schedule. In this series of experiments, FR x was varied with FR y held constant at both high and low values. In addition, FR y was varied with FR x held constant at both high and low values. The results indicate that the post-prime pause is primarily a function of FR y, the number of responses required after the priming run. On the other hand, both post-reinforcement pauses were shown to be a function of FR x, the size of the small fixed ratio. An interaction between FR x and FR y and both the post-reinforcement and the post-prime pauses suggested that pausing in mixed schedules is a closed system. Taken as a whole, these results indicate the importance of mixed chained schedules in the investigation of the priming phenomenon

Intensity of Grief and Belief in Personal Control

Much research has focused on stress and its consequences. Some studies have indicated personal control helps an individual handle stress better and remain healthier. Little research, however, has examined the role personal control plays in bereavement as a stressful life event This correlational study examined the relationship between belief in personal control and grief intensity experienced from losing a loved one to death. Volunteer subjects primarily from Grief Support Groups completed the Belief in Personal Control Scale and the Texas Revised Grief Inventory Results demonstrated some instances when higher belief in personal control resulted in lower intensity of grief in the present, when variables of time since death and mode of death were considered Lack of belief in personal control was shown to be a risk factor for poor outcome in some instances

The Fixed-Interval Presentation of a Small Fixed Ratio

In mixed fixed-ratio schedules, reinforcement is delivered according to two or more fixed-ratio components, which may be programmed in simple alternation or according to a random sequence. If the components simply alternate, the behavior is similar to that obtained in a multiple schedule. Pauses are long before long fixed ratios and short before short fixed ratios. If a random sequence is used, responding is characterized by short pauses after reinforcement independent of the size of the ratio and primes occur at the beginning of many of the fixed ratios. The purpose of the present study was to investigate the possibility of other patterns of responding being controlled by component sequences ether than simple or random alternation in mixed fixed-ratio schedules. Two pigeons were trained to key peck on a fixed-ratio 100 (FR 100) schedule. Following this training, the schedule was based on a superimposed fixed-interval contingency which specified that an FR 10 would occur after the completion of the first FR 100 following a fixed period, timed from the last FR 10. Thus, the schedule was a mixed FR 100 FR 10 with FR 10 frequency controlled by the superimposed fixed-interval contingency. The value of the FI was varied for both birds. To control for the possibility that the pattern of responding was related to priming, two additional pigeons were trained to respond on comparable multiple schedules in which primes were absent. The mixed schedule shortened the pre-FR 100 pause. Analysis of the sequence of pauses following an FR 10 showed that the pause following an FR 10 was long relative to other pauses in the sequence which were short and approximately equal to the pre-FR 10 pause. Primes, a run of approximately ten responses at the beginning of a ratio followed by a pause, were relatively infrequent in the first FR 100 after an FR 10, but were more frequent in other positions in the sequence. In the multiple schedule, all pre-FR 100 pauses were long and approximately ·equal in length, but the pre-FR 10 pauses were short. These results demonstrate that the pattern of responding obtained in the mixed schedule was related to priming. First, short pauses preceded FR 100s which contained primes in the mixed schedule. Primes did not occur in the multiple schedule, and the pre-FR 100 pauses were long. Second, primes were frequent only in those ordinal positions in the mixed schedule which were preceded by short pauses. Third, short pauses and primes developed simultaneously. These three effects suggest a new account of priming as a unit of behavior. This unit consists of a short pause followed by a run of ten responses. This run of ten responses is intermittently reinforced by food delivery when the FR 10 occurs. The pause after this run, which occurs if the schedule is FR 100, is caused by the S-delta condition produced by the information that the schedule is not FR 10. The pattern of responding in the mixed schedule was also related to the sequential nature of the schedule. Since FR 100s always followed FR 10s, FR 10s were followed by long pauses and few primes. FR 100s, on the other hand, were occasionally followed by FR 10s. Thus, the completion of an FR 100 was a discriminative stimulus for FR 10-like behavior

Intensity of Grief and Belief in Personal Control

Much research has focused on stress and its consequences. Some studies have indicated personal control helps an individual handle stress better and remain healthier. Little research, however, has examined the role personal control plays in bereavement as a stressful life event. This correlational study examined the relationship between belief in personal control and grief intensity experienced from losing a loved one to death. Volunteer subjects primarily from Grief Support Groups completed the Belief in Personal Control Scale and the Texas Revised Grief Inventory Results demonstrated some instances when higher belief in personal control resulted in lower intensity of grief in the present, when variables of time since death and mode of death were considered. Lack of belief in personal control was shown to be a risk factor for poor outcome in some instances

Psychological factors related to Buruli ulcer and tuberculosis in Sub-Saharan Africa

We onderzochten de relatie tussen ziektepercepties en hulpzoekgedrag, onder volwassenen die in risicogebieden wonen voor de infectieuze huidaandoening Buruli ulcus uit Benin, en onder tuberculose patiënten uit Nigeria. Ziektepercepties bleken gerelateerd te zijn aan het tijdstip waarop mensen hulp zochten. Mensen die Buruli ulcus als een chronische aandoening zagen, de behandeling effectief vonden, en veel controle ervoeren over hun aandoening hadden een grotere kans om pas in een later stadium hulp te zoeken. Tuberculose patiënten richtten zich vaak eerst tot twee of drie andere behandelvormen voordat ze een gespecialiseerd centrum bezochten. Vertraging was geassocieerd met werkeloosheid en negatieve emoties. Daarnaast werd pijn tijdens de behandeling van Buruli ulcus onderzocht. Hoewel Buruli ulcus beschreven wordt als een pijnloze aandoening, laten klinische observaties zien dat er wel degelijk pijn is. Patiënten werden gevraagd naar hun pijn, en hulpverleners werden geïnterviewd over hun attitudes ten aanzien van pijn. Het bleek dat patiënten een lage pijnscore rapporteerden, en dat pijn afnam over de tijd. Een gedeelte van de patiënten (30%) rapporteerde hoge pijnscores. 2 patiënten kregen pijnmedicatie. Dus, in tegenstelling tot de literatuur, ervaren patiënten wel degelijk pijn. Tot slot werd de psychometrische kwaliteit van twee vragenlijsten onderzocht, namelijk een lijst die depressieve symptomen meet ‘CES-D´ en een lijst die persoonlijke controle meet; de Pearlin mastery schaal. We vonden dat 17 van de 20 items informatief waren in het meten van depressieve symptomen in onze West-Afrikaanse steekproef. 3 items waren minder informatief. Bij de Pearlin mastery schaal waren vier van de zeven items informatief