Observations, which are an integral part of any science, can be classified into two main types: qualitative and quantitative. In general, qualitative observations are non-numeric. As a result they are not exact and are often not reproducible (two people generally do not describe a particular color in exactly the same way). Reproducibility is an important aspect of science and so it is preferable to use quantitative observations or measurements to describe an experiment. Measurements are made with instruments that allow us to exceed the limitations of our senses. Some instruments, such as a ruler that can be used to measure length, or a balance that can be used to measure mass, are simple to use. Other more complicated instruments, such as a spectrophotometer (used for measuring the relative intensities of light in different parts of the spectrum), or a nuclear magnetic resonance (NMR) machine...