Yohanan Wax

Estimating medical costs from incomplete follow-up data.

Estimation of the average total cost for treating patients with a particular disease is often complicated by the fact that the survival times are censored on some study subjects and their subsequent costs are unknown. The naive sample average of the observed costs from all study subjects or from the uncensored cases only can be severely biased, and the standard survival analysis techniques are not applicable. To minimize the bias induced by censoring, we partition the entire time period of interest into a number of small intervals and estimate the average total cost either by the sum of the Kaplan-Meier estimator for the probability of dying in each interval multiplied by the sample mean of the total costs from the observed deaths in that interval or by the sum of the Kaplan-Meier estimator for the probability of being alive at the start of each interval multiplied by an appropriate estimator for the average cost over the interval conditional on surviving to the start of the interval. The resultant estimators are consistent if censoring occurs solely at the boundaries of the intervals. In addition, the estimators are asymptotically normal with easily estimated variances. Extensive numerical studies show that the asymptotic approximations are adequate for practical use and the biases of the proposed estimators are small even when censoring may occur in the interiors of the intervals. An ovarian cancer study is provided.

Production of Macrobrachium rosenbergii in monosex populations: Yield characteristics under intensive monoculture conditions in cages

Abstract The production of Macrobrachium rosenbergii monosex populations was examined under intensive growth conditions in cages. It was found that an all-male population yielded 473 g/m 2 within 150 days, whereas an all-female population and a mixed population produced 248 g/m 2 and 260 g/m 2 , respectively, during the same growout period. Since the performance of the mixed population in cages is comparable to that of a normal typical juvenile population in commercial earthen ponds, with an average production of 2500–3000 kg/ha, it appears possible to obtain a yield of 4700 kg/ha within 150 days from all-male populations grown under intensive monoculture conditions. A comparison of growth performance of each of the sexes when raised alone and when raised in mixed populations showed that whereas females are strongly affected by the presence of males, resulting in growth inhibition, males are hardly influenced by the presence of females. The findings of this study call for intensive efforts in research directed towards the controlled establishment of prawn monosex populations.

Regression analysis of grouped survival data : informative censoring and double sampling

To analyze grouped survival data subject to informative censoring, we propose the following two-part model: a logistic regression model for the hazard for failure, given covariates and a logistic regression model for the hazard for informative censoring, given time of failure and covariates. We fit the model to survival data arising from a double sampling design: In a full follow-up (FF) sample subjects are followed after censoring, and in a partial follow-up (PF) sample subjects are not followed after censoring. We illustrate the methodology using data from a study of wound infection in which patients in the PF sample are censored by release from the hospital, whereas patients in the FF sample are followed after hospital release. We discuss identifiability when there is only a PF sample. We also consider how the allocation between the PF and FF samples affects the precision of estimates.

Possible use of Brachionus plicatilis (O.F. Müller) as food for freshwater cyprinid larvae

Abstract The use of the euryhaline rotifer Brachionus plicatilis as food in raising freshwater fish larvae was tested with the Japanese ornamental carp, koi ( Cyprinus carpio ) and a breed of bait fish ( Carassius sp.). Providing rotifers in addition to granulated food improved significantly the growth rate of fish larvae and also, in the case of ornamental koi, their survival. Growth rate in terms of increase in length was found to fit a simple empirical formula similar to a power growth curve. The curve fits results obtained with bait fish ( R 2 =0.85) better than those with Japanese koi ( R 2 =0.76). The coefficients of growth for each treatment ( β i ) were used to evaluate the contribution of rotifers to the larval growth. The improvement in growth was found to correlate with the quantity of rotifers, being higher with the larger quantity offered (2400 larva −1 day −1 ). In bait larvae, results obtained from measuring length were supported in weight measurements taken on day 8, but not by those recorded on day 11. Between day 8 and day 11, these larvae were fed on granulated food only. Preliminary results indicate that yeast-fed rotifers were superior to algae-fed ones when supplied to bait fish larvae in equal quantities, in addition to granulated food.

Evaluation of a Supermarket Intervention

A 2-year supermarket intervention in 20 pair-matched stores tested whether a nutrition education program could produce changes in food purchasing behavior. Food sales data for 2 intervention years were compared with baseline year data. The evaluation presented challenges related to the quasi-experimental study design and to the high variability and auto- and cross-sectional correlations in the data. A complex time series model was used to take these factors into account and to test for intervention effects.

Factors affecting survival of rotifers (Brachionus plicatilis O.F. Müller) at 4°C

Abstract The possibility of maintaining rotifers ( Brachionus plicatilis O.F. Muller) at 4°C for extended periods of time at high densities and at a low cost was examined. It was found that the dynamic processes involved in the maintenance of a rotifer population (hatching and production of eggs) continue to operate at 4°C, but their rate is reduced to one-tenth of that occurring at 25°C. The survival of rotifers and the percent of eggs carried by them were affected by the frequency of changing the culture media, the salinity in which they were cultured, and the amount of food provided and its type (algae, yeast or Topal). Higher survival rates were observed in rotifers maintained in the dark. Statistical analysis revealed that in most cases, the time spent at 4°C significantly affected both survival and the percent of eggs. On several occasions, time interacted with the tested effect, that is, there were differences in the direction of the trend in time between the tested groups. In such a case, the contribution of each of the studied effects could not be evaluated over the total period of the experiments. It had to be analysed separately for each of the observation days; the most relevant being the last day of the experiment. Results of the present research indicate that the cheapest system would involve maintaining rotifers (at least 1000 ml −1 ) cultured in 10‰ seawater at 4°C with yeast as food. Media would have to be changed every 4–8 days, and the cultures should be maintained in the dark. These rotifers may be used directly, following proper enrichment with unsaturated fatty acids, as food for marine fish larvae, or alternatively to initiate new cultures. The present paper proposes a new method for the preservation of rotifers for periods extending to days or weeks. This would allow more flexibility in managing the supply and demand in a marine hatchery. Alternatively, stored rotifers could be distributed from one main location to small hatcheries or other users lacking facilities for culturing rotifers.

The Israeli Study of Surgical Infections (ISSI): II. Initial comparisons among hospitals with special focus on hernia operations.

In a study of 5,571 patients from the general surgery departments of 11 Israeli hospitals, the crude overall wound infection rates showed interhospital heterogeneity. The rates ranged from 6.3% to 12.4% (P(chi 2) = 0.039). Controlling for the different distributions of procedures performed in the various institutions did not reduce this variability. None of the hospitals had either consistently high or consistently low infection rates. A hospital could have low rates for one procedure and high rates for another. Therefore, the decision was made to proceed with procedure-specific analyses. This article details results of the analysis of 1,487 hernia operations. Four variables (old age, infection at another site, incarceration, and introduction of drains) accounted for almost all the differences in infection rates among the institutions. Of the four, presence of drains had the strongest association with infection (P derived from the logistic model less than 0.001). The risk was consistent in all hospitals and was unconfounded by other measurable factors. In contrast, the pattern of using drains seemed arbitrary and inconsistent, ranging from 9% of patients in one hospital to 41% in another. These findings were used as a basis for discussion with the surgical teams and for the initiation of a randomized clinical trial on the use of drains in hernia operations.

Evaluation of the relative inhibitory potential of fractionated urinary macromolecules

SummaryUltrafiltration membranes of 10,000 d, 1,000 d and 500 d were used to remove urinary macromolecules from the urine of normal subjects and from the urine of stone forming patients. The filtrated urines were examined for their residual inhibitory potential for calcium-oxalate precipitation, by the discrimination method of Sarig et al. (D.I. test). The results of testing the filtrate were complementary to the information gained by analyses of retentates obtained in successive ultrafiltration. The method has an inherent advantage because the manipulation of solids retained on membranes may inadvertantly modify their inhibitory potential. At least two distinct groups of inhibitors were found in 20 normal urines. The first group has MW above 10,000 d while the second group of inhibitors has MW in the range of 500–1,000 d. The mean of the D.I. values increased dramatically from the normal range (<0.6) to the stone former range (>1.1) (p<0.001) after the 500 d filtration. Some of the normal urines, even after the 500 d filtration, still had a degree of inhibitory potential. This inhibitory potential may be related to the inorganic compounds which were found in the urines. The inhibitory activity of macromolecules with MW above 10,000 d and below 500 δ was negligible in 7 stone formers (SF) urines. The relative contribution of 500–1,000 d macromolecules is the highest both in SF and normal urines. Conclusions: 1) inhibitors in human urine are of wide range in MW; 2) stone formers and normals differ in the level of inhibitor activity at all MW ranges; especially in above 10,000 d and below inhibitors.

Epidemiological trends of dermatophytoses and dermatophytes in Jerusalem between 1954 and 1981

Data for dermatophyte infections analysed for five 3-year periods between 1954 to 1981 led to the following conclusions: (1) Tinea pedis, tinea cruris and tinea manuum showed an increase in the 50s and 60s and declined in the 70s; (2) Tinea unguium and tinea corporis showed an increase during the whole period; (3) At all these sites, the percentage of Trichophyton rubrum, the main etiologic agent, increased steadily over the periods while the percentage of Trichophyton mentagrophytes, the secondary etiological agent, decreased. Epidermophyton floccosum, the third etiological agent in these sites, showed no sharp fluctuations; (4) These three dermatophytes which show similar microclimatic requirements and favour the same microecological niches, were called ‘glabrohydrophilic’. In tinea corporis they form a definite subset, their percentage being similar to that at other glabrous sites; (5) Tinea capitis was at its peak in the 50s, decreased sharply until the second half of the 70s, its main etiological agent being Trichophyton violaceum. Since 1979, an increase of tinea capitis occurred due to the newly introduced Microsporum canis; (6) Dermatophytes favouring scalp hair were called ‘trichophilic’. In tinea corporis they form a definite subset, their percentage being similar to that of tinea capitis; (7) A comparison with other studies from this country shows that macroclimate (i.e. humid warm coastal climate compared with dry cooler inland-mountain climate) is not an important factor in the etiology of tinea.

Cholesterol uptake capacity of Acholeplasma laidlawii is affected by the composition and content of membrane glycolipids.

The composition of the cell membrane of 20 Acholeplasma laidlawii strains grown under identical conditions was studied and correlated with the capacity of these strains to incorporate cholesterol. Membranes of these strains had similar sodium dodecyl sulfate-polyacrylamide gel electrophoresis patterns and contained the same lipid species, but the relative amounts of the major polar lipids varied. Statistical analyses revealed that the glycolipids, monoglucosyldiglyceride, and an unidentified glycolipid (glycolipid-X) succeeded in explaining 90% (R2 = 0.90) of the cholesterol uptake variations. The regression coefficients for both glycolipids were negative (P less than 0.001), indicating that the capacity of A. laidlawii strains for cholesterol incorporation is inversely proportional to the relative amounts of these glycolipids. Accordingly, an increased capacity for cholesterol incorporation was detected upon aging of A. laidlawii cells. The aged cells contained significantly smaller amounts of both monoglucosyldiglyceride and glycolipid-X, and a higher amount of diglucosyldiglyceride. The change in cholesterol incorporation as a response to glycolipid composition and content can be explained by the low solubility of cholesterol in glycolipids as well as by the induction by the sterol molecule of a nonlamellar phase state that will destabilize a membrane structure containing monoglucosyldiglyceride and glycolipid-X.