(It is possible to find negative values for reliability correlations, but when this happens something is seriously, seriously wrong.) There is no one standard for acceptable reliability, but. It is enough to know that Pearson correlation coefficients of reliability nearly always range between 0 and 1.00. There's no need to explain here how it is computed you can look that up if you like. In psychological measurement we like to quantify the amount of reliability of a test with a statistic called the Pearson correlation coefficient.

As physics developed more reliable methods of measurement, we have been able to improve the measurement precision and accuracy to enable remarkable technological achievements, from producing nuclear energy to connecting the world through the Internet to safely flying more than 8 million people through the sky each day. Modern physics has depended on various standards of distance, defining a meter in 1960 as the distance between two ends of a particular platinum-iridium bar stored under controlled conditions and in 1983 as the distance traveled by light in a vacuum in 1 / 299,792,458th of a second. 1120 the king of England declared that the standard of length would be called a yard, defined by the distance from the tip of his nose to the end of his outstretched arm. These standards have changed over of the history of measurement.

For physical properties, this problem has been successfully handled by simply defining the three basic units of measurement (length, mass, and time) according to agreed-upon standards.

Reliability in psychological assessment