Research process involves several steps and each step depends on the preceding steps. If step is missing or inaccurate, then the succeeding steps will fail. When developing research plan, always be aware that this principles critically affects the progress. One of critical aspects of evaluation and appraisal of reported research is to consider the quality of the research instrument. According to Parahoo (2006), in quantitative studies reliability and validity are two of the most important concept used by researcher to evaluate the quality of the study that is carried out. Reliability and validity in research refer specifically to the measurement of data as they will be used to answer the research question. In most cases, the instrument that measures the variable is the central issues in determining the reliability and validity of the data. Whatever data collection method is used, the intent must be accuracy and the result of the research is depend on the consistency, stability and repeatability of data collection instrument or in other word its reliability.
In addition to reliability, researcher need to know if the measurement technique used to collect data actually measures what it is supposed to measure; in other word is it a valid technique?. Estimating the degree to which an instrument is valid and reliable is a critical step in the research process because this determines how much weight can be placed on the result (Wood & Rose-Kerr, 2006). The concept of reliability and validity will be discussed as they ultimately will influence the data analysis and the final report.
The reliability of research instrument is a major criterion for assessing its quality. An instrument’s reliability is the consistency with which it measures the target attribute. According to Polit & Beck (2008), the definition of reliability is the accuracy and consistency of information obtained in study. Supported by Parahoo (2006), reliability refers to the consistency of particular method in measuring or observing the same phenomena. Reliability means the accuracy of the data in the sense of stability and repeatability. Reliability refers to the extent to which the questionnaire would produce the same results if used repeatedly with the same group under the same conditions. A reliable instrument maximizes the true score component and minimizes the error component of an obtained score.
The less variation an instrument produces in repeated measurement, the higher its reliability (Polit & Beck, 2012). Thus, reliability can be equated with a measure’s stability, consistency, or dependability. There are two ways of explaining reliability (consistency and accuracy) are not so different as they might appear. Errors of measurement that impinge on an instrument’s accuracy also affect its consistency. There are three method of testing the reliability of research instrument: test for the stability of the instrument (how stable it is over time), internal consistency (the measurement of the concept is consistent in all parts of test) and test for equivalence (consistency of the result by different investigators or similar test at the same time). Each test of reliability looks at a different aspect of the instrument.
3.1 Test of Stability
Stability is the best indicator of an instrument’s reliability. According to Wood & Rose-Kerr (2006), a stable research instrument is one that can be repeated over and over on the same research subject and will produce the same results. Testing for stability only can be done on stable or constant variable such as intelligence. It should be possible to measure intelligence repeatedly, at regular interval, and to obtain the same score. An unstable concept such as pain, on the other hand, is changeable and repeated measure of pain in a subject would result in widely different score. Although stability is good indicator of reliability when the...