If the temperature T does not vary too much, a linear approximation is typically used: R ( T ) = R 0 [ 1 + α α ( T − − T 0 ) ] where α α is called the temperature coefficient of resistance, T 0 is a fixed reference temperature (usually room temperature), and R 0 is the resistance at temperature T 0 . The parameter α α is an empirical parameter fitted from measurement data. Because the linear approximation is only an approximation, α α is different for different reference temperatures. For this reason it is usual to specify the temperature that α α was measured at with a suffix, such as α α 15 , and the relationship only holds in a range of temperatures around the reference.