摘要：Indicators are natural, every where, part of everyone's life. Instinctively we all use indicators to monitor systems we care about or need to control e.g. mothers are alert to the activity level of their children, the brightness of their eyes, the way they breathe in sleep. The learning of every school child is expressed as test scores and grades. Farmers scan the sky for weather fronts, squeeze the soil to measure its moisture, watch how many earthworms are turned over in a shovelful of earth. Doctors take your temperature; look at your tongue, do blood tests and check hart bits. Mechanics use calipers and pressure gauges and listen to the sound of the motor. Pilots and power plant operators have whole panels of instruments in front of them. Economists use leading indicators, lagging indicators, cost-of-living indicators, employment indicators, or index and all indicators, the GDP.
