抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Much previous research has established that perceived ease of use is an important factor influencing user acceptance and usage behavior of information technologies. However, very little research has been conducted to understand how that perception forms and changes over time. The current work presents and tests an anchoring and adjustment-based theoretical model of the determinants of system-specific perceived ease of use. The model proposes control (internal and external--conceptualized as computer self-efficacy and facilitating conditions, respectively), intrinsic motivation (conceptualized as computer playfulness), and emotion (conceptualized as computer anxiety) as anchors that determine early perceptions about the ease of use of a new system. With increasing experience, it is expected that system-specific perceived ease of use, while still anchored to the general beliefs regarding computers and computer use, will adjust to reflect objective usability, perceptions of external control specific to the new system environment, and system-specific perceived enjoyment. The proposed model was tested in three different organizations among 246 employees using three measurements taken over a three-month period. The proposed model was strongly supported at all points of measurement, and explained up to 60% of the variance in system-specific perceived ease of use, which is twice as much as our current understanding. Important theoretical and practical implications of these findings are discussed.

/pdf/determinants-of-perceived-ease-of-use-integrating-control-10opxseq0g.pdf

Determinants of Perceived Ease of Use: Integrating Control, Intrinsic Motivation, and Emotion into the Technology Acceptance Model

Collaborative filters help people make choices based on the opinions of other people. GroupLens is a system for collaborative filtering of netnews, to help people find articles they will like in the huge stream of available articles. News reader clients display predicted scores and make it easy for users to rate articles after they read them. Rating servers, called Better Bit Bureaus, gather and disseminate the ratings. The rating servers predict scores based on the heuristic that people who agreed in the past will probably agree again. Users can protect their privacy by entering ratings under pseudonyms, without reducing the effectiveness of the score prediction. The entire architecture is open: alternative software for news clients and Better Bit Bureaus can be developed independently and can interoperate with the components we have developed.

http://pages.cpsc.ucalgary.ca/~saul/601.13/readings/p175-resnick.pdf

GroupLens: an open architecture for collaborative filtering of netnews

We develop a new index of economic policy uncertainty (EPU) based on newspaper coverage frequency Several types of evidence – including human readings of 12,000 newspaper articles – indicate that our index proxies for movements in policy-related economic uncertainty Our US index spikes near tight presidential elections, Gulf Wars I and II, the 9/11 attacks, the failure of Lehman Brothers, the 2011 debt-ceiling dispute and other major battles over fiscal policy Using firm-level data, we find that policy uncertainty raises stock price volatility and reduces investment and employment in policy-sensitive sectors like defense, healthcare, and infrastructure construction At the macro level, policy uncertainty innovations foreshadow declines in investment, output, and employment in the United States and, in a panel VAR setting, for 12 major economies Extending our US index back to 1900, EPU rose dramatically in the 1930s (from late 1931) and has drifted upwards since the 1960s

/pdf/measuring-economic-policy-uncertainty-4nrg3ei76f.pdf

Measuring Economic Policy Uncertainty

https://repository.upenn.edu/cgi/viewcontent.cgi?article=1056&context=fnce_papers

Expected stock returns and volatility

How should firms decide whether and when to invest in new capital equipment, additions to their workforce, or the development of new products? Why have traditional economic models of investment failed to explain the behavior of investment spending in the United States and other countries? In this book, Avinash Dixit and Robert Pindyck provide the first detailed exposition of a new theoretical approach to the capital investment decisions of firms, stressing the irreversibility of most investment decisions, and the ongoing uncertainty of the economic environment in which these decisions are made. In so doing, they answer important questions about investment decisions and the behavior of investment spending.This new approach to investment recognizes the option value of waiting for better (but never complete) information. It exploits an analogy with the theory of options in financial markets, which permits a much richer dynamic framework than was possible with the traditional theory of investment. The authors present the new theory in a clear and systematic way, and consolidate, synthesize, and extend the various strands of research that have come out of the theory. Their book shows the importance of the theory for understanding investment behavior of firms; develops the implications of this theory for industry dynamics and for government policy concerning investment; and shows how the theory can be applied to specific industries and to a wide variety of business problems.

Investment Under Uncertainty

First course in Econometrics in Economics Departments at better schools, also Economic/Business Forecasting. Statistics prerequisite but no calculus. Slightly higher level and more comprehensive than Gujarati.

Econometric Models and Economic Forecasts

Most investment expenditures have two important characteristics: First, they are largely irreversible; the firm cannot disinvest, so the expenditures are sunk costs. Second, they can be delayed, allowing the firm to wait for new information about prices, costs, and other market conditions before committing resources. An emerging literature has shown that this has important implications for investment decisions, and for the determinants of investment spending. Irreversible investment is especially sensitive to risk, whether with respect to future cash flows, interest rates, or the ultimate cost of the investment. Thus if a policy goal is to stimulate investment, stability and credibility may be more important than tax incentives or interest rates. This paper presents some simple models of irreversible investment, and shows how optimal investment rules and the valuation of projects and firms can be obtained from contingent claims analysis, or alternatively from dynamic programming. It demonstrates some strengths and limitations of the methodology, and shows how the resulting investment rules depend on various parameters that come from the market environment. It also reviews a number of results and insights that have appeared in the literature recently, and discusses possible policy implications.

https://papers.ssrn.com/sol3/Delivery.cfm/nber_w3307.pdf?abstractid=226671&mirid=1

Irreversibility, Uncertainty, and Investment

Despite its importance to economic growth and the evolution of market structure, the investment behavior of firms, industries, and countries remains poorly understood. This paper has several objectives. First, it reviews some basic models of irreversible investment to illustrate the option-like characteristics of investment opportunities, and to show how optimal investment rules can be obtained from methods of option pricing, or alternatively from dynamic programming. Second, it discusses the implication of irreversibility for the empirical analysis of investment behavior. Finally, it discusses briefly some of the implications that the irreversibility of investment may have for policy. For example, policies that stabilize prices or exchange rates may be effective ways of stimulating investment.

/pdf/irreversibility-uncertainty-and-investment-1bngqxxntj.pdf

A model of capacity choice and utilization is developed consistent with value maximization when investment is irreversible and future demand is uncertain. Investment requires the full value of a marginal unit of capacity to be at least as large as its full cost. The former includes the value of the firms option not to utilize the unit, and the latter includes the opportunity cost of exercising the investment option. We show that for moderate amounts of uncertainty, the firm's optimal capacity is much smaller than it would be if investment were reversible, and a large fraction of the firm's value is due to the possibility of future growth. We also characterize the behavior of capacity and capacity utilization, and discuss implications far the measurement of marginal cost and Tobin's q.

Robert S. Pindyck

Papers

Investment Under Uncertainty

Econometric Models and Economic Forecasts

Irreversibility, Uncertainty, and Investment

Irreversibility, Uncertainty, and Investment

Irreversible Investment, Capacity Choice, and the Value of the Firm