The Nines System is the informal name for a grading scale often used at educational institutions in English-speaking countries, particularly the United States.
The system owes its name to the fact that each of the top four letter grades in it cover a range of nine points. The minimum passing mark under it is almost always 65%, or five points higher than in the more widely-used Tens System.
The simplest variant of the Nines System is shown below:
92 to 100 = A
83 to 91 = B
74 to 82 = C
65 to 73 = D
Below 65 = F
The option exists to append "+" and "−" signs to each of the letters (but customarily not to the "F"), splitting each of the top four letter grades into three subdivisions, covering a span of three points each as nine is evenly divisible by three. This yields the following possible grades:
98 to 100 = A+
95 to 97 = A
92 to 94 = A−
89 to 91 = B+
86 to 88 = B
83 to 85 = B−
80 to 82 = C+
77 to 79 = C
74 to 76 = C−
71 to 73 = D+
68 to 70 = D
65 to 67 = D−
Below 65 = F
An "F+" grade will sometimes be added, often with a range of 60 to 64, and less commonly an "F−" will exist as well.
At some institutions the "A+" grade is not recognized, usually due to the fact that the grade point average system is in use; when this scenario arises, the "A+" range shown above may merely be lumped in with the "A," or a more even division of the "A" category may be observed, for example by reckoning 96 to 100 as "A" and 92 to 95 as "A−."
The Eights System is the informal name for one of the three most common grading scales used at educational institutions in the United States.
The distribution is uneven because with the minimum passing standard of 70% the system observes, there are 31 possible passing numerical grades — from 70 through 100, all inclusive.
As in other popular grading systems, a "+" or "−" modifier may be added to any of the letters (but customarily not to the "F") in order to draw a finer distinction.
A system may be considered highly reliable (that is, it may fail very infrequently), but if it is out of service for a significant period of time as a result of a failure, it will not be considered highly available.
System designers can enhance the fault tolerance of a system by combining simple hardware redundancy with fault-management software: replicating hardware subsystems to provide the necessary back-up hardware in the case of failure and executing a copy of the same software on each node provide simple hardware redundancy.
Otherwise, system designers and engineers will be forced to make a platform choice that may not be suited to their needs: they will be precluded from choosing alternative platforms that would work better for their application, or provide cost or other competitive advantages.
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