Necessary Condition For Validity Research Articles

Diagnosing Insensitivity to Scope in Contingent Valuation

Sensitivity to scope is considered a desirable property of contingent valuation studies and often treated as a necessary condition for validity. We first provide an overview of scope insensitivity explanations put forth in the environmental valuation literature. Then we analyze data from a contingent valuation survey eliciting willingness-to-pay to prevent oil spills of four different magnitudes in Arctic Norway. In the baseline analysis, the scope inference is ambiguous. There is only statistical difference in willingness to pay to avoid a very large versus small oil spill (NOK 1869 and NOK 1086, respectively). However, further explorations show that several confounding factors suggested in the literature influence the scope inference. The scope sensitivity improves when we control for subjective probabilities of amenity provision, exclude respondents based on the debriefing questions, take into consideration the sample sizes, and impose diminishing marginal utility. Overall, the analysis supports an emerging view in the contingent valuation literature suggesting that statistical scope insensitivity is not a sufficient reason for deeming a study invalid.

TU‐F‐17A‐09: Four‐Dimensional Cone Beam CT Ventilation Imaging Can Detect Interfraction Lung Function Variations for Locally Advanced Lung Cancer Patients

Purpose:Four‐dimensional cone beam CT ventilation imaging (4D‐CBCT VI) is a novel functional lung imaging modality requiring validation. We hypothesize that 4D‐CBCT VI satisfies a necessary condition for validity: that intrafraction variations (e.g. due to poor 4D‐CBCT image quality) are substantially different to interfraction variations (e.g. due to changes in underlying function). We perform the first comparison of intrafraction (pre/post fraction) and interfraction (week‐to‐week) 4D‐CBCT VIs for locally advanced non small cell lung cancer (LA NSCLC) patients undergoing radiation therapy.Methods:A total of 215 4D‐CBCT scans were acquired for 19 LA NSCLC patients over 4‐6 weeks of radiation therapy, including 75 pairs of pre‐/post‐fraction scans on the same day. 4D‐CBCT VIs were obtained by applying state‐of‐the‐art, B‐spline deformable image registration to obtain the Jacobian determinant of deformation between the end‐exhale and end‐inhale phases. All VIs were deformably registered to the corresponding first day scan, normalized between the 10th and 90th percentile values and cropped to the ipsilateral lung only. Intrafraction variations were assessed by computing the mean and standard deviation of voxel‐wise differences between all same‐day pairs of pre‐/post‐fraction VIs. Interfraction differences were computed between first‐day VIs and treatment weeks 2, 4 and 6 for all 19 patients. We tested the hypothesis by comparing cumulative distribution functions (CDFs) of intrafraction and interfraction ventilation differences using two‐sided Kolmogorov‐Smirnov goodness‐of‐fit tests.Results:The (mean ± std. dev.) of intrafraction differences was (−0.007 ± 0.079). Interfraction differences for weeks 2, 4 and 6 were (−0.035 ± 0.103), (−0.006 ± 0.094) and (−0.019 ± 0.127) respectively. For week 2, the changes in CDFs for intrafraction and interfraction differences approached statistical significance (p=0.099).Conclusion:We have shown that 4D‐CBCT VI can satisfy a necessary condition for validity; intrafraction variations do not limit the ability to measure interfraction variations in LA NSCLC patients.This work is supported by a Cancer Institute NSW Early Career Fellowship, an NHMRC Australia Fellowship and NIH P01CA116602.

Validity of the Michaelis–Menten equation – steady‐state or reactant stationary assumption: that is the question

The Michaelis-Menten equation is generally used to estimate the kinetic parameters, V and K(M), when the steady-state assumption is valid. Following a brief overview of the derivation of the Michaelis-Menten equation for the single-enzyme, single-substrate reaction, a critical review of the criteria for validity of the steady-state assumption is presented. The application of the steady-state assumption makes the implicit assumption that there is an initial transient during which the substrate concentration remains approximately constant, equal to the initial substrate concentration, while the enzyme-substrate complex concentration builds up. This implicit assumption is known as the reactant stationary assumption. This review presents evidence showing that the reactant stationary assumption is distinct from and independent of the steady-state assumption. Contrary to the widely believed notion that the Michaelis-Menten equation can always be applied under the steady-state assumption, the reactant stationary assumption is truly the necessary condition for validity of the Michaelis-Menten equation to estimate kinetic parameters. Therefore, the application of the Michaelis-Menten equation only leads to accurate estimation of kinetic parameters when it is used under experimental conditions meeting the reactant stationary assumption. The criterion for validity of the reactant stationary assumption does not require the restrictive condition of choosing a substrate concentration that is much higher than the enzyme concentration in initial rate experiments.

Concordance and consistency of answers to the self‐delivered ESPAD questionnaire on use of psychoactive substances

Considering the prevalence of drug use in Italy, it is crucial to develop a reproducible screening test. Test-retest reliability and internal consistency are important indicators of a measurement's temporal stability and are a necessary condition for validity. The aim of the study was to assess the consistency and concordance of the European School Survey Project on Alcohol and Other Drugs (ESPAD) questionnaire; participating students completed the questionnaire twice, with a three-week interval. To verify the concordance for variables relating to use of alcohol, cigarettes and illicit drugs, the original ordinal variables as well as the same dichotomically recodified variables were used. Data analysis was done using Kappa and weighted Kappa. The method proposed by Lipsitz was used to evaluate the influence of gender and age on concordance. Questions about drug use, examined in ordinal form, show a good test-retest concordance and an excellent concordance for answers relating to the use of cigarettes, alcohol and cannabis. Regarding the effect of age adjusted for gender, 15-year-old subjects showed a lower concordance than 19-year-olds. ESPAD is a tool with a good reproducibility. Results focus on the effect of gender and age covariates on the concordance of answers regarding drug use and suggest the importance of examining the concordance in relation to the covariate levels.

An evaluation of the reliability of probability judgments across response modes and over time

AbstractDespite the importance of probability assessment methods in behavioral decision theory and decision analysis, little attention has been directed at evaluating their reliability and validity. In fact, no comprehensive study of reliability has been undertaken. Since reliability is a necessary condition for validity, this oversight is significant. The present study was motivated by that oversight. We investigated the reliability of probability measures derived from three response modes: numerical probabilities, pie diagrams, and odds. Unlike previous studies, the experiment was designed to distinguish systematic deviations in probability judgments, such as those due to experience or practice, from random deviations. It was found that subjects assessed probabilities reliably for all three assessment methods regardless of the reliability measures employed. However, a small but statistically significant decrease over time in the magnitudes of assessed probabilities was observed. This effect was linked to a decrease in subjects overconfidence during the course of the experiment.

Cause-deleted proportional mortality analysis and the healthy worker effect.

This paper evaluates the relationships between various indices of proportional mortality, used in occupational and other settings where denominator data are unavailable. In particular, we examine the necessary conditions for validity of the proportional mortality ratio (PMR) and mortality odds ratio (MOR) as estimates of the standardized mortality ratio (SMR), in general and for the situation where one considers only a subset of deaths, with certain causes of death deleted from the data. We may exclude data in this way to avoid the inflationary effect on the PMRs for some causes of death (e.g. cancer) that results from a 'healthy worker effect' for other causes of death (e.g. heart disease). We find that cause-deleted PMRs and MORs may be unbiased estimates of their corresponding SMR, and these indices appear most useful for specific hazards of employment or relatively infrequent causes of death.

Thermal radiation from pool fires

For an axisymmetric, horizontal, pool fire of specified flame shape, effective flame radiation (Schmidt) temperature and a gray flame absorption coefficient, this analysis computes (1) radiative energy fluxes to surfaces located external to the fire in any arbitrary orientation, (2) variations of radiative heat flux along the fuel surface, from fire center to fire edge, (3) the total radiative heat transfer from the flames to the fuel surface, (4) forward radiative heat transfer from the fire to the virgin fuel bed external to the fire, (5) the angular distribution of the radiative flux emitted by the pool fire and (6) the total radiative power output of the fire. The calculations agree with experimentally measured radiative fluxes at different locations on the pool surface and outside the fire. The radiative flux from the flames to the burning fuel surface is maximum at the center of the fuel bed and it decreases markedly toward the edge of the fire. The forward radiative heat transfer from the flames, to the virgin fuel bed external to the fire, is highest at the leading edge of the fire and it decays rapidly with increasing distance from it. Necessary conditions for validity of isotropic flame radiation are established.