OCR Text |
Show The fine probe functions as follows: 1. The volunteers are arranged the same as for the coarse probe, with 30- inch spacing center to center. 2. Each volunteer probes in front of his left foot, then in the center of his straddled position, and finally in front of his right foot. ( See Figure 16.) 3. On signal, the line advances one foot and repeats the three probes. 4. The exact number of signals from the probe line leader is arbitrary, but the fine probe usually functions best when each probe pole insertion and retraction is performed on command. 5. Good discipline and coordinated probing is even more necessary than with the coarse probe. Careless or irregular probing can negate the advantages of fine probing. Use of the stringline for a guide is especially important with the fine probe. The three insertions are made along the line of the guide string, which is then moved ahead one foot to pace the probe line. Effectiveness of the coarse and fine probes can be compared by examining the relation of sampling ( probe insertion) intervals to the projected area of a human body. The result is that the coarse probe technique has a 76% chance of locating a victim on a given pass, while the fine probe has essentially a 100% chance of locating the victim. 2.4 Optimizing the Victim's Chances of Survival If the coarse probe gives only a 76% chance of finding the victim compared with 100% for the fine probe, why is it preferred as the initial search measure? Basic probability theory provides the answer. As mentioned earlier, the successful rescue operation is the one which gives the greatest probability of finding the vict im alive. The coarse probe is not completely thorough, but it is fast. In fact, experience shows it is four to five times faster than the fine probe. A glance at Figure 1 shows the dependence of the victim's life on a speedy rescue. When gain of speed is weighed against loss of completeness, calculations show that the victim's chances of survival are greatly improved if the search operation begins with a coarse probe. ( Reference No. 8.) Suppose the initial coarse probe is unsuccessful. Should the coarse probe be repeated, or should the next step be a fine probe? This question is analogous to the coin- tossing problem: A coin has been tossed and it turns up " heads," which it has a 50% chance of doing. What is the probability it will turn up heads on the next toss? The answer is still 50%, demonstrating the well- known principle that " the laws of chance have no memory." 39 |