Saatnya Sekarang

Background :  I am trying to evaluate the effect on traffic safety of a fog warning system deployed in California in November 1996.  The system was installed by CalTrans on a section of I-5 and SR-120 near Stockton where the accident rate is generally high, particularly during the morning commute when ground fog is common.   The warning system consists of (1) weather monitoring stations that detect fog and (2) changeable message signs that warn drivers to reduce speed. I will post my findings as I go in order to solicit comments from professionals and demonstrate methods for students.  If I can get permission, I will also post my data and code so you can follow along at home. Previously : In the previous installment I reviewed the first batch of data I'll work with, and ran some tests to confirm that Poisson regression is appropriate for modeling the number of accidents in a given day. Poisson regressions Traffic volume To measure the effect of traffic volu...

I've started a new project, working with a collaborator at another university, to evaluate the impact of a fog warning system deployed on a highway in California in November 1996.  The system was installed by CalTrans on a section of I-5 and SR-120 near Stockton where the accident rate is generally high, particularly during the morning commute when ground fog is common. The warning system consists of (1) weather monitoring stations that detect fog and (2) changeable message signs that warn drivers to reduce speed.  Among people who study traffic safety, there are two theories about these kinds of systems: 1) The mainstream theory is that when drivers are warned to slow down, they slow down, and lower speeds reduce the accident rate. 2) The heterodox theory, which my collaborator holds, is that warning signs introduce perturbations into the flow of traffic so they can cause more accidents than they prevent. My job is to evaluate which theory the data support.  Here...

Excessive use of the passive voice in science writing is a self-perpetuated, mutually-perpetrated hoax.  To prove it, I am offering a $100 bounty for the first person who can find a journal that explicitly requires authors to write in the passive voice. UPDATE April 4, 2012: To my shock and dismay, the bounty has been claimed!  See the new HALL OF SHAME below. Most style guides recommend the active voice, and most readers prefer it. But in some academic fields, especially the sciences, authors use a stilted and awkward style that replaces clear concise sentences like, "We performed the experiment," with circumlocutions like "The experiment was performed." Asked why they write like that, many scientists admit that they don't like it, but they are under the impression that journals require it.   They are wrong.   Of the journals that have style guides, the vast majority explicitly ask authors to write in the active voice. ...

I am always looking for good examples of Bayesian analysis, so I was interested in this paragraph from The Economist (September 2000): "The canonical example is to imagine that a precocious newborn observes his first sunset, and wonders whether the sun will rise again or not. He assigns equal prior probabilities to both possible outcomes, and represents this by placing one white and one black marble into a bag. The following day, when the sun rises, the child places another white marble in the bag. The probability that a marble plucked randomly from the bag will be white (ie, the child’s degree of belief in future sunrises) has thus gone from a half to two-thirds. After sunrise the next day, the child adds another white marble, and the probability (and thus the degree of belief) goes from two-thirds to three-quarters. And so on. Gradually, the initial belief that the sun is just as likely as not to rise each morning is modified to become a near-certainty that the sun will alway...