VALIDITY OF MEASUREMENT S P M V Subbarao Professor Mechanical Engineering Department Justification for Selection of Concepts to Hardware ?????

How To Measure Any Other Property? It is essential to invent a scientific principle, which connects the property to be measured and physical displacement/length/ Just a Real Number. One needs to identify/ develop hardware which can work as per the scientific principle. What is the guarantee that the hardware exactly works as per the principle? How to develop high degree of confidence in a measurement?

Validity of Measurement When we decide to study a variable, we need to devise some way to measure it. Some variables are easy to measure and others are very difficult. We try to develop the best measures we can whenever we are doing research. A good measuring instrument or test is one that is reliable and valid. Let us look at test validity first.

Test Validity Test Validity refers to the degree to which a measuring strategy (instrument, machine, or test) measures what is to be measured. This sounds obvious; right? A valid measure is the one that accurately measures the variable being studied. There are four/five ways to establish that your measure is valid: Content validity Construct validity Predictive validity Concurrent validity Convergent validity and/or Discriminant validity.

Content Validity Content validity is established if your measuring instrument samples from the areas of skill or knowledge that compose the variable. This assumes that you have a good detailed description of the domain, something that's not always true. More the number of valid theories/skills, more will be the number of measurement strategies. Consider measurement of temperature: Most popular valid theory for construction is ?

Coefficient of Thermal Expansion material CTE (ppm/°C) silicon3.2 alumina6–7 copper16.7 tin-lead solder27 E-glass54 S-glass16 epoxy resins15–100 silicone resins30–300 It is important to realise that: The CTE is often not the same in all axes (that is, not ‘isotropic’). The CTE is rarely linear. The variation in CTE with temperature is only a fairly smooth function if the material is undergoing no phase transitions.

Construct validity is the approximate truth of the conclusion that the measurement accurately reflects truth. The degree of translation of property to be measured into the measure. Construct validity is based on designing a measure that logically follows from a theory or hypothesis. Predictive validity, assesses the measurement's ability to predict something it should theoretically be able to predict. Concurrent validity, assesses the measurement's ability to distinguish between groups that it should theoretically be able to distinguish between. Refers to the ability of any measure to separate subjects who possess the attribute being studied from those who do not. Convergent validity assesses the degree to which the measurement is similar to (converges on) other measurements that it theoretically should be similar to. It is used when a valid measure exists for your variable but you want to design another measure that is perhaps easier to use or faster to take. Discriminant validity, examines the degree to which the measurement is not similar to (diverges from) other measurement that it theoretically should be not be similar to.

Ideas of Measurement Validation

Reliability Reliability is the consistency with which our measure measures. If you cannot get the same answer twice with your measure it is not reliable. A measuring strategy can be reliable and not valid, but if the instrument is not reliable it is also not valid. Measurement is never exact. At some point our measures always break down and errors creep into our data. This is when the concept of Error of Measurement becomes important. In order to be able to use any measure we need to know its error of measurement.

A good measuring strategy is reliable and, because it is reliable, it has a small amount of error in its observations. The First Law of Measurements