การศึกษาแผนแบบการเลือกตัวอย่างและขนาดตัวอย่างที่เหมาะสม เพื่อใช้ประมาณจำนวนกลุ่มไข่หนอนเจาะลำต้นข้าวโพด / สายพิณ กาญจนเสริม = A study of sampling design and optimal sample size on the estimation of the eggs mass of corn stem rober / Saipin Karnjanasirm
For several years the Department of Agriculture has studied whether or not to spray insecticide on corn to kill corn stem borer. The department has now come to the following conclusions : If we find masses of eggs on corn stalks and leaves which amounts 15 eggs masses/100 plants due to the percentage of parasitization (60-80%) by the Trichogramma sp. parasite, then we should spray insecticide immediately. (Auranuj Kongkanjana, et al., 2525). The reason we check the masses of eggs is that if we spray insecticide the mass of eggs is less than 15 egg masses/100 plants, the insecticide could kill parasitization of corn stem borer such as Chrysopa basalis Walker, Proreus similans Slallen and Trichogramma australicum Girault. The result in this case is that the following year the corn stem borer will increase because of the lack of its natural enemies and the corn stem borer itself will become resistant to the insecticide which means that we must increase the concentration of the insecticide next time. Counting the mass of eggs requires a lot of experience. Therefore, if we can find a process of sampling and selecting good samples which can approximate the total of the mass of eggs with a 2 egg masses/100 plants tolerance, then this will be very helpful to the farmers. From our research, we have found that the distribution of the egg mass is in one of the forms of binomial, poisson or negative binomial. The test have that been used are the tests for “goodess of fit” and “t-test”. These tests have shown that the data in B.E. 2525 had a negative bionomial distribution or clump distribution (Poole, 1974 and Southwood, 1978) in one period. That period was when the corn was 30 days of age. However when the corn was 75 days of age, the distribution of mass of eggs is poisson distribution or random distribution. (Poole, 1974 and Southwood, 1978). From the age of 45 days and 60 days, the distribution is unknown. In B.E. 2526, most of the corn distribution from the sampling had poisson, except when the corn was 5 weeks old. Using this set of data, we formulated the hypothesis that the distribution will be poisson distribution. This hypothesis would be accepted if the α is equal to 0.01 but would not be accept if the α is equal to 0.05. From this tests, we found that distribution could be in the form of negative bionomial or poisson or bionomial. Sometimes the distribution is not in any know form at all. The samples of the corn stalks that were used in the approximation of the net amount of the egg mass were selected as follows. When the age of corn was 5 to 6 weeks, there was a lot of egg mass on the corn stalks. The samples were about 5.5 percent of the whole corn in that lot. From the age of 9 to 10 weeks, these was less egg mass and the samples were around one percent of the corn in that lot. However an infection of only 3.5 percentages of the corn stalks in the lot will be enough for all other ages. The techniques in sampling the corn for the approximation of the egg mass are selected in such a manner that the result is as close as possible to the exact amount. The sampling techniques used, depend upon the age of the corn as the follows. At the age of 2 weeks, the technique in sampling should be area sampling. In this case the grid size should be 2.25 lines of 4.5 meters. When the corn reaches the age of 3 weeks, the sampling technique is still area sampling with a new grid size of 3.75 meters by 3 meters. A new technique, sample sampling, is used with corn at age of 4 weeks. The area sampling is used again when the corn is reaches the age of 5 weeks using a grid size of 2.25 meters by 4.5 meters. At the age of 6 weeks the sampling technique is changed to systematic sampling. When the corn is 7 to 8 weeks old, a two stage sampling technique is used instead. At 9 weeks of age, the sampling technique is the area sampling with a grid size of 3 times 3 meters. At 10 weeks of age, the two stage sampling is used. From the above, it can be seen that the techniques that are used in sampling are different during different growth periods. So, for the purpose of this study in handling the sampling process and teaching the gardener to understand the process, the simple random sampling and the systematic sampling are the methods selected. Both techniques are provided in this case in practicing and also give good results because their approximations are lesser different from the exact percentage than any other techniques.
