Author: Robert Hyatt
Date: 21:22:58 09/08/02
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On September 08, 2002 at 19:52:18, Ricardo Gibert wrote: >On September 07, 2002 at 11:13:20, Robert Hyatt wrote: > >> >>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... :) > >You have to compute run_time too: run_time = completion_time - start_time ;-) Maybe. One could argue that you start run-time at zero, such as a CPU clock cycle counter, then you don't have to do the calculation. But remember, I am not claiming these are not "measured" values. But on the pure definition of the term "observed" there is an issue.
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