Entirely possible overruns: how people think and talk about probabilistic cost estimates

Purpose The purpose of this paper is to examine people’s understanding and evaluation of uncertainty intervals produced by experts as part of a quality assurance procedure of large public projects. Design/methodology/approach Three samples of educated participants (employees in a large construction company, students attending courses in project management and judgment and decision making, and judges of district and appeal courts) answered questionnaires about cost estimates of a highway construction project, presented as a probability distribution. Findings The studies demonstrated additivity neglect of probabilities that are graphically displayed. People’s evaluations of the accuracy of interval estimates revealed a boundary (a “cliff”) effect, with a sharp drop in accuracy ratings for outcomes above an arbitrary maximum. Several common verbal phrases (what “can” happen, is “entirely possible” and “not surprising”) which might seem to indicate expected outcomes were regularly used to describe unlikely values near or at the top of the distribution (an extremity effect). Research limitations/implications All judgments concerned a single case and were made by participants who were not stakeholders in this specific project. Further studies should compare judgments aided by a graph with conditions where the graph is changed or absent. Practical implications Experts and project managers cannot assume that readers of cost estimates understand a well-defined uncertainty interval as intended. They should also be aware of effects created by describing uncertain estimates in words. Originality/value The studies show how inconsistencies in judgment affect the understanding and evaluation of uncertainty intervals by well-informed and educated samples tested in a maximally transparent situation. Readers of cost estimates seem to believe that precise estimates are feasible and yet that costs are usually underestimated.

Supraorbital Keyhole Craniotomy for Basilar Artery Aneurysms: Accounting for the “Cliff” Effect

BACKGROUND: Treatment of basilar artery aneurysms is challenging. While endovascular techniques have dominated, there still remain circumstances where open surgical clipping is required or preferred. Minimally invasive “keyhole” approaches are being used more frequently to provide the durability of surgical clipping with a lower morbidity profile; however, careful patient selection is required. The supraorbital “keyhole” approach has been described for the treatment of basilar artery aneurysms, but careful assessment of the basilar exposure is necessary to ensure proper visualization of the aneurysm and ability to obtain proximal vascular control. Various methods of estimating the basilar artery exposure in this approach have been described, including the anterior skull base line and the posterior clinoid line, but both are unreliable and inaccurate. OBJECTIVE: To propose a new method, the orbital roof-dorsum line, to simply and accurately predict the basilar artery exposure. METHODS: CT angiograms for 20 consecutive unique patients were analyzed to obtain the anterior skull base line, posterior clinoid line, and the orbital roof-dorsum line. CT angiograms were then loaded onto a Stealth neuronavigation system (Medtronic, Minneapolis, Minnesota) to obtain “true” visualization lengths. A case illustration is presented. RESULTS: Pairwise comparison tests demonstrated that both the anterior skull base and the posterior clinoid estimation lines differed significantly from the “true” value (P < .0001). Our orbital roof-dorsum estimation provided results that accurately predicted the “true” value (P = .71). CONCLUSION: The orbital roof-dorsum line provides a simple and reliable method of estimating basilar artery exposure and should be used whenever considering patients for surgical clipping by this approach.

Is More Precise Dating of Paleoindian Expansion Feasible?

Recent efforts to precisely date the florescence of the Clovis culture in North America have been hampered by both practical and theoretical problems: 1) The era of Clovis expansion (about 11,200–10,700 BP or 13,200–12,700 cal BP) coincides with the gap between the anchored central European tree-ring sequence (back to 12,400 cal BP) and the floating Bølling-Allerød sequence; 2) Clovis seems to immediately precede the onset of the Younger Dryas (YD) stadial. The “black mats” of the US Southwest appear to mark the regional occurrence of this climatic downturn. However, the timing and means of long-distance propagation of this climatic event are not yet well understood. Greenland ice cores (GISP2, GRIP, and NGRIP) remain poorly synchronized, with a discrepancy of 100 to 250 yr for the date of onset (as late as 12,700 cal BP, or as early as 12,950 cal BP); 3) The YD onset was accompanied by a rapid drop of radiocarbon ages from 11,000 to 10,600 BP in less than a century. The mechanism causing this was probably a change in overturning circulation in the North Atlantic. Do variable Clovis ages, often from what appear to be single-occupation contexts, reflect this “cliff” effect, slightly earlier minor reversals during the late Allerød, or simply the practical limitations of precision of the 14C method? 4) Dates for Fishtail or Fell I sites (with fluted, stemmed points) in southern South America are statistically indistinguishable from Clovis dates in North America. Does this imply very rapid population expansion, diffusion of tool-making techniques through long-established local populations (as argued by Waters and Stafford 2007), or abnormally large interhemispheric 14C offsets? 5) Are recent ostensibly high-precision collagen-derived dates for Paleoindian-associated fauna (e.g. horse and mammoth) reliable? Are interlaboratory blind tests of the new filtration processes necessary?