The International Rice Research Institute in the Philippines developed the hybrid rice IR 8, setting off "the green revolution" in tropical agriculture. Among other things, they made a thorough study of the effects of fertilizer on rice yields. These experiments involved a number of experimental plots (of about 20 square yards in size). Each plot was planted with IR 8, and fertilized with some amount of nitro-

gen chosen by the investigators. (The amounts ranged from 0 to about a pound.)

When the rice was harvested, the yield was measured and related to the amount of nitrogen applied. In one such experiment, the correlation between rice yield and nitrogen was 0.95, and the regression equation was

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predicted rice yield = (20 oz rice per oz nitrogen) x (nitrogen) + 240 oz.

(a) An unfertilized plot can be expected to produce around ______ of rice.

(b) Each extra ounce of nitrogen fertilizer can be expected to increase the rice yield by ______.

(c) Predict the rice yield when the amount of fertilizer is 3 ounces of nitrogen 4 ounces of nitrogen

(d) Was this an observational study or a controlled experiment?

(e) In fact, fertilizer was applied only in the following amounts: 0 ounces, 4 ounces, 8 ounces, 12 ounces, 16 ounces. Would you trust the prediction for 3 ounces of nitrogen, even though this particular amount was never applied?

(f) Would you trust the prediction for 100 ounces of nitrogen? pl875