Measurement instruments such as questionnaires are used for measuring variables. Levels of measurement refer to the degree of precision with which that level supposedly measures variables. Nominal is the most basic and merely names the variables. Ordinal comes next and adds the characteristic of putting the variables in rank order. Interval builds on both of these criteria and adds the criteria of equal value of intervals.
Ratio includes the previous three with the addition of an absolute zero.
Reliability is the most important concept involved in measurement. Reliability refers to the consistency for which the measurement instrument measures what is says it measures. Validity in measurement refers to the degree that the measurement instrument measures what it says it measures. Three types of validity are content, criterion, and construct. Using Data Collection Instruments Questionnaires use many types of questions.