Observation is either an activity of a living being (such as a human), consisting of receiving knowledge of the outside world through the senses, or the recording of data using scientific instruments. The term may also refer to any datum collected during this activity.

The scientific method

The scientific method requires observations of nature to formulate and test hypotheses. It consists of these steps:

1. Asking a question about a natural phenomenon
2. Making observations of the phenomenon
3. Hypothesizing an explanation for the phenomenon
4. Predicting a logical consequence of the hypothesis
5. Testing the prediction in a controlled experiment, a natural experiment, an observational study, or a field experiment
6. Creating a conclusion with data gathered in the experiment

Observation plays a role in the second and fifth steps. However the need for reproducibility requires that observations by different observers be comparable. Human sense impressions are subjective and qualitative making them difficult to record or compare. The idea of measurement evolved to allow recording and comparison of observations made at different times and places by different people. Measurement consists of using observation to compare the thing being measured to a standard; an artifact, process or definition which can be duplicated or shared by all observers, and counting how many of the standard units are comparable to the object. Measurement reduces an observation to a number which can be recorded, and two observations which result in the same number are equal within the resolution of the process.

Human senses are limited, and are subject to errors in perception such as optical illusions. Scientific instruments were developed to magnify human powers of observation, such as weighing scales, clocks, telescopes, microscopes, thermometers, cameras, and tape recorders, and also translate into perceptible form events that are unobservable by human senses, such as indicator dyes, voltmeters, spectrometers, oscilloscopes, interferometers, geiger counters, MRI machines, radio telescopes, and DNA sequencers.

One problem encountered throughout scientific fields is that the observation may affect the process being observed, resulting in a different outcome than if the process was unobserved. For example, it is not possible to check the air pressure in an automobile tire without letting out some of the air, changing the pressure. However, in most fields of science it is possible to reduce the effects of observation to insignificance by using better instruments.

Considered as a physical process itself, all forms of observation, human and instrumental, involve amplification and are therefore thermodynamically irreversible processes, increasing entropy.

Alternate definitions

In some specific fields of science the words 'observer' and 'observation' have to be redefined to take into account factors that are unimportant in everyday observation: