The uncertainty of scale reading is a measure of how well an instrument scale can be read. Repeated measurements can help to decrease random uncertainty. Systematic uncertainties occur when all of the readings are either too little or too large. They can't be eliminated, but they can be quantified by repeating the measurement process. Random errors are due to limitations in the measuring instruments themselves. For example, a thermometer may have a 0.5°C margin of error. This means that if a thermometer reads 100.0°F, then the true temperature may be anywhere from 99.5°F to 101.5°F. In general, statistical measures such as standard deviation or coefficient of variation can be used to quantify random errors. Systematic errors are those that remain even after repeated measurements have been made. Examples include an instrument's zero point error and its linearity error. Zero point error refers to the amount by which a given instrument offsets other readings it takes simultaneously with itself. For example, an electrical voltage meter might show 9 volts even though it is actually set at 8.9 volts. Its zero point error would be 0.1 volt. Linearity error refers to the degree to which a measuring instrument distorts actual readings when operated within certain limits. For example, a voltmeter will usually display 5 volts even though only 4.9 volts is being sent from a power source.