Quantitative observation is any kind of observation that can be expressed numerically as a measured value. A few simple examples of quantitative observations are temperature, time and mass, but any observation that can be expressed as a specific number is quantitative. Quantitative observations are useful because they can easily be compared to one another. With enough of these observations, scientists can make useful predictions about the world.
Quantitative observation is key to scientific research in fields ranging from physics to sociology, although researchers in the physical sciences most commonly employ it. Quantitative data are useful in part because they can be analyzed statistically. This analysis can, among other things, help determine the significance of research. Scientists can use a statistical tool called a chi-squared test to determine the probability that the results they have obtained are a result of random chance. For example, if a scientist rolls a die 20 times and, after reviewing the results, feels that it might be loaded, he can use a chi-squared test to determine the probability that those results can be derived randomly from a fair die.
Quantitative observation can also help scientists make predictions about the world. For example, in chemistry, scientists can time a reaction at varying reactant concentrations to determine how those concentrations affect the overall reaction rate. With enough quantitative observation, they can effectively predict the rates of reaction in untested concentrations.