Research instruments fall into two broad categories: those compiled by the researcher him or herself in the form of recorded observations, logs, and rating scales and those completed by the interview subject him or herself in the form of questionnaires and interviews. Regardless of the instruments used, research studies should be guided by acceptable standards of validity and reliability. "Validity is the extent to which an instrument measures what it is supposed to measure and performs as it is designed to perform" ("Instrument, validity, reliability" 2014). Statistical tests are usually used to establish the external validity of an instrument. "External validity is the extent to which the results of a study can be generalized from a sample to a population" ("Instrument, validity, reliability" 2014). In contrast, internal validity is the extent to which the instrument is internally valid based upon the results obtained within the sample. An instrument may be said to be reliable if it consistently produces the same results when tested upon the population it is intended...

"The degree to which different raters/observers give consistent answers or estimates" establishes its inter-rater reliability ("Instrument, validity, reliability" 2014). An instrument can be valid (i.e., tests what it is supposed to test) but not be reliable; it can also produce reliable results yet not be valid.
Another distinguishing feature of research studies is that of the population vs. The sample. A population is an entire group of people vs. The part of the population or sample about which generalizations can be extrapolated which apply to the larger population. Sampling techniques can be random -- i.e., random in nature in the sense that anyone can be a member of the sample group. In contrast, with "stratified random sampling, the researcher first divides the population into groups based on a relevant characteristic and then selects participants within those groups. In educational research, stratified random sampling is typically used when the researcher wants to ensure that specific subgroups of people are adequately…