Reliability in research means that an instrument yields the same results again and again on every trial. There may be difference in the results; therefore we use more and less reliable instrument for a specific research. A more reliable instrument is often called as stable instrument (that instrument which gives same or very closer results) where as less reliable instrument is often called unstable instrument (that gives different results on every trial). Reliability of an instrument = More same results of an instrument trialed every time Joppe (2000) defines reliability as:
“…The extent to which results are consistent over time and an accurate representation of the total population under study is referred to as reliability and if the results of a study can be reproduced under a similar methodology, then the research instrument is considered to be reliable.” Joppe, M. (2000). The Research Process. Retrieved February 25, 1998.
Validity in research context refers to the extent to which a test measures what we actually wish to measure. In other words validity means the results of a research instrument relates to our criteria. When the results and criterion are unrelated then the instrument is invalid, and should not be used in research further. Validity = relatedness of results with what we wish to observe Joppe (2000) defines validity as:
“Validity determines whether the research truly measures that which it was intended to measure or how truthful the research results are. In other words, does the research instrument allow you to hit "the bull’s eye" of your research object? Researchers generally determine validity by asking a series of questions, and will often look for the answers in the research of others.” Joppe, M. (2000). The Research Process. Retrieved February 25, 1998.
Golafshani, N. (2003). Understanding reliability...