Reliability and Validity of measuring Instrument and its Importance

Reliability:  Reliability means consistency with which the instrument yields similar results. 

Validity: Validity is the ability of a measuring instrument to actually measure what it claim to measure.  

These are basic concept of reliability and Validity.

  In Urban development research, the common term for any type of measurement device is instrument.
Thus the instrument could be a test, scale, questionnaire, interview schedule etc. 
In development research, one of the most important criteria for the quality of measurement is reliability of the measuring instrument.

The inherent aspects and synonyms of reliability are : - 

  • Dependability 
  • Stability   
  • Consistency 
  • Predictability 
  • Accuracy 
  • Equivalent 

Reliability means consistency with which the instrument yields similar results. Reliability concerns the ability of different researchers to make the same observations of a given phenomenon if and when the observation is conducted using the same method and procedure. 

The subject of validity is complex and very important research because it is in this more than anywhere else, that the nature of reliability of questioned. 

It is possible to study reliability without inquiring into the nature and meaning of one's variable. 

Validity as the extent to which any measuring instrument measures what it is intended to measure. 

But, Strictly speaking one validates not a measuring instrument, but an interpretation of data arising from a specific procedure. 
Thus, one validates not the measuring instrument itself, but the measuring instrument in relation to the purpose for which it is being used. 

There are four approaches to validation of measuring instruments -  

  1. Logical Validity/ Face Validity 
  2. Jury Opinion 
  3. Known-Group
  4. Independent criteria
The real difference between reliability and validity  is mostly a matter of definition Reliability estimates the consistency of your measurement. Validity, on the other hand, involves the degree to which you are measuring what you are supposed to.