Instrumental variables estimation

Summary

In statistics, econometrics, epidemiology and related disciplines, the method of instrumental variables (IV) is used to estimate causal relationships when controlled experiments are not feasible or when a treatment is not successfully delivered to every unit in a randomized experiment. Intuitively, IVs are used when an explanatory variable of interest is correlated with the error term, in which case ordinary least squares and ANOVA give biased results. A valid instrument induces changes in the explanatory variable but has no independent effect on the dependent variable, allowing a researcher to uncover the causal effect of the explanatory variable on the dependent variable. Instrumental variable methods allow for consistent estimation when the explanatory variables (covariates) are correlated with the error terms in a regression model. Such correlation may occur when:

changes in the dependent variable change the value of at least one of the covariates ("reverse" causation),

there

Official source

https://en.wikipedia.org/wiki/Instrumental_variables_estimation

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Getting cited: does open access help?

Patrick Gaulé

We reexamine the widely held belief that free availability of scientific articles increases the number of citations they receive. Since open access is relatively more attractive to authors of higher quality papers, regressing citations on open access and other controls yields upward-biased estimates. Using an instrumental variable approach, we find no significant effect of open access. Instead, self-selection of higher quality articles into open access explains at least part of the observed open access citation advantage.

2008

The first chapter of this thesis, coauthored with Annamaria Conti, seeks to add an economic contribution to the current debate on using university licensing contracts to improve access to medicines in developing countries. We build a simple model in which we have a university licensing out an academic invention to a profit-maximizing pharmaceutical company. We compare three different types of licensing contracts that the university might use to enhance access to pharmaceuticals in the South: (1) an exclusive license limited to the North; (2) an exclusive license worldwide with a price cap in the South; and (3) an exclusive license worldwide with a price cap in the South and a clause specifying that the licensee would lose its exclusivity in the South if it does not supply the Southern market. We show that in a simple model with asymmetric information on production costs the latter type of contract dominates the two others. In the second chapter, coauthored with Nicolas Maystre, we reexamine the widely held belief that free availability of scientific articles increases the number of citations they receive. Since open access is relatively more attractive to authors of higher quality papers, regressing citations on open access and other controls yields upward-biased estimates. Using an instrumental variable approach, we find no significant effect of open access. Instead, self-selection of higher quality articles into open access explains at least part of the observed open access citation advantage. The third chapter uses an evidence-based approach to assess the difficulties faced by developing country scientists in accessing the scientific literature. I compare backward citations patterns of Swiss and Indian scientists in a database of 43'150 scientific papers published by scientists from either country in 2007. Controlling for fields and quality with citing journal fixed effects, I find that Indian scientists (1) have shorter references lists (2) are more likely to cite articles from open access journals and (3) are less likely to cite articles from expensive journals. The magnitude of the effects is small which can be explained by informal file sharing practices among scientists.

EPFL2009

Programming and use of Parylene C fluorescence as a quantitative on-chip reference

Ata Tuna Ciftlik, Diego Gabriel Dupouy, Martinus Gijs, Joan Teixidor

A large number of lab-on-a-chip applications use fluorescence for quantifying biological entities. In such applications, incorporation of a stable on-chip fluorescent reference source would be highly desirable in order to compensate for instrumental parameter variations, like for example non-constant illumination intensity over time. In this study, we exploit Parylene C that is used as a bonding layer material in a microfluidic chip. We first show programming of intermediate Parylene C bonding layer fluorescence (iPBLF) and its characterization as a function of the ultraviolet (UV) dose and Parylene C thickness. This technique requires no additional steps in the fabrication of the microfluidic chip and the fluorescence reference zones can simply be incorporated by local exposure to UV light after fabrication. Next, we demonstrate a fluorescence-based analyte concentration and flow-rate measurement in a microfluidic channel, under changing experimental conditions of illumination intensity and taking different microscope objectives. Sensing is realized by analyzing programmed reference and channel images by straightforward data handling using an open-source image processing tool. We anticipate that the demonstrated method will be a key technique allowing low complexity and reliable quantitative fluorescent measurements, for example in point-of-care and mobile diagnostic applications, where intensity calibration can present a major challenge.

Royal Soc Chemistry2014

EPFL2009