Author: Robert Hyatt
Date: 08:13:20 09/07/02
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Jose made a really good point about observed data vs measured data. After thinking about it for a bit, I decided that it is a point strong enough to change the way we think about "measured" and "observed". Some examples: speed. Impossible to measure. For example, your automobile (newer vehicles) compute speed by counting the revolutions of the tailshaft (output) of the transmission, then factoring in the rear-end ratio and the circumference of the rear wheels. It _computes_ the speed from that. A radar measures the frequency change in a radio signal as it bounces off a moving target and _computes_ the speed based on the frequency change. A GPS observes to "positions" in terms of lattitude and longitude, uses some geometry to compute the distance between them, and uses a clock to measure the time to cover that distance, and displays speed. So Speed can't be measured directly, it has to be computed. And this isn't a surprise since speed is defined as distance over time. Brightness. (of a light, not a person. :) ) This is a direct measure of an electrical signal produced by some sort of device (photo-resistor, photo-cell, optical transistor, etc) and then that voltage is used to compute a brightness level in Lumens... Loudness (sound). Ditto. NPS. nodes searched divided by time in seconds. Computed. Speedup one-processor time divided by the N-processor time. Computed We really don't have a lot of "observed" data nowadays. Some, yes. Where were you at 8pm last night. But more is computed... Which means if we start to define observed vs computed, we don't end up with very much in the "observed" column. In a chess program I can count nodes and "compute" time (end-time minus start-time) and then compute a nps value. I can measure run-time and compute speed-up. But I can't directly measure speed at all. strange when you think about it... :)
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