Where does crash collide?

A sudden, severe fall in the prices of traded assets, the kind marked by dates like 1929 or Black Monday in 1987. In Stock-Market Crashes and Depressions (2017), Barro and Ursúa define a crash operationally as a cumulative real drop in a stock-price index of 25% or more, and ask whether such falls forecast depressions in the wider economy. The referent is an asset market's price level, and the unit is the fractional drop in an index over a short window. What crashes here is a number that stands for value, not a machine and not a body. Nothing halts, nothing collides, and nothing dies; a price simply falls fast and far.[1]

Unit

The fractional fall in an asset-price index over a short window

Recovery means

Applicable: prices can rebound, though not always to the prior peak

Measured by

The cumulative real return of a price index over a short window, for example a drop of 25% or more

The abrupt failure in which a running program or machine stops functioning rather than continuing to deliver its service. In Basic Concepts and Taxonomy of Dependable and Secure Computing (2004), Avizienis, Laprie, Randell, and Landwehr define a failure as the event when delivered service departs from correct service, and a crash is the mode in which a component halts. Schneider's 1990 tutorial Implementing Fault-Tolerant Services Using the State Machine Approach makes the semantics precise: a crash-failing component simply stops, cleanly, rather than producing wrong output. The referent is a running process, and the unit is a single halting event. No price falls and no object is struck; the system just quits.[2][3]

Unit

A discrete halting event in a running program or machine

Recovery means

Applicable: a crashed process is typically restarted into a clean state

Measured by

Observed as an abrupt halt, where the component stops delivering service instead of returning wrong output

A sudden collapse in the number of individuals in a population, usually after it has overshot the resources that sustain it. Klein's 1968 study The introduction, increase, and crash of reindeer on St. Matthew Island follows a herd that grew from 29 animals to roughly 6,000 and then fell to 42 in a single hard winter once it had stripped its food supply. The referent is the abundance of a biological population, and the unit is a count or density of living individuals. What crashes is a headcount of organisms, not a market index and not a machine. The collapse plays out over a season or a few generations, and the animals that die do not restart or rebound the way a price or a process can.[4]

Unit

The number of individuals in a biological population

Recovery means

Applicable: a remnant may recover, or the population may go locally extinct

Measured by

Census counts or density estimates falling sharply over one or a few generations