# What is the difference between a measurement and an experiment?

For example, say I want to determine the atomic structure of a bio molecule. I purify the molecule, get it to crystallize (probably though trial and error), shoot it with X-rays, observe the scatter amplitudes, determine the scatter phases through an optimization procedure involving prior data, determine atomic coordinates, and possibly do additional refinement until I have the final structure. Is this a measurement of the structure or an experiment to determine it? Or something else?

Maybe it's an issue of linguistics, I'm not sure. Perhaps a measurement just gives a number, but an experiment answers a question based on a measurement? If x-ray crystallography is an experiment, what is the question it's answering?

• On one single molecule you measure all the data you said? Is the molecule very big? More exactly, is it a classical object? I guess that you take an ensemble of such molecules for obtaining all those data. In this case a measurement is what you do with one molecule, and the whole experiment contains many measurements. Mar 11, 2015 at 0:04
• The answer to this depends on whether or not it is the first time you are determining the atomic structure of any bio molecule. If it is, then I'd call this an experiment. If, on the other hand, this is an established procedure you have for the routine determination of atomic structure, then it is not an experiment. If the process is so well defined that you could, in theory, write a program to automate the entire process, then this becomes a measurement. If it is less refined and requires you to do more of the intermediate steps, then it is an analysis of the atomic structure
– Jim
Apr 18, 2017 at 12:43

Measurements are a field of their own. One measures the time, the temperature outside the window, the wind velocity, the length of a skirt etc . A measurement is a real number associated with a single reading on a measuring instrument which is calibrated to give real numbers as answers to specific questions: what is the time, how hot is it ...etc.

An experiment gets real numbers as an answer , but it presupposes a hypothesis , a theory, which asks a question based on the hypothesis, uses instrumentation and intermediate physical processes, and gets real numbers that describe the objective of the hypothesis.

In your molecular example the hypothesis is that there exists a structure and the desire is to get concrete numbers for this structure, using many measurements.

A physics experiment is based on many measurements. A measurement is a single reading on a calibrated instrument.

• 'A physics experiment is based on many measurements. A measurement is a single reading on a calibrated instrument.' so at e.g., LHC, we only measure energy deposites in calorimeter cells etc? (or something even more basic). We can't e.g. 'measure' the Higgs mass? or the Z-mass? etc? Apr 18, 2017 at 12:53

An experiment could refer to the steps undergone to get a certain physical quantity.So it may be right to state that a measurement is obtained after a set of experiments are carried out.

My take on this is that an experiment requires a manipulation of nature such as to produce a minimum set of conditions necessary to approximate a given phenomenon. Often this means a simplification of the natural case. Experiments may therefore involve a reduction in the complexity of a natural phenomenon in order to render it's workings more evident. The experimenter also has a choice of how to conduct his experiment (experimental design) and so can tune it to suit a given research question

A measurement (or an observation of a natural phenomenon) can thus be thought of as the full Monty, i.e. no simplification.

I don't think there is a widely-accepted distinction between performing an experiment versus performing a measurement. In that context, they may be interchanged without a change in meaning. Note, however, that an experiment, may refer to a laboratory and its scientific program, such as ATLAS and CMS experiments at the LHC.

I would personally say, though, that a distinction may be drawn with help from statistical language:

• In a measurement, one performs parameter inference. One estimates quantities from observations. E.g., measurements of the Higgs boson mass at the LHC.
• In an experiment, one performs hypothesis tests or model selection. One attempts to determine the best model for explaining observations. E.g., rejection of the Standard Model without a Higgs boson at the LHC.