Additionally, many schools add .33 for a + grade and subtract .33 for a − grade. Thus, a B+ yields a 3.33 whereas an A– yields a 3.67. A+s, if given, are usually assigned a value of 4.0 (equivalent to an A) due to the common assumption that a 4.00 is the best possible grade-point average, although 4.33 is awarded at some institutions. In some places, .25 or .3 instead of .33 is added for a + grade and subtracted for a − grade. Other institutions maintain a mid-grade and award .5 for the grade. For example, an AB would receive a 3.5-grade point and a BC would receive a 2.5-grade point. The industry standard for graduation from undergraduate institutions is a minimum 2.0 average. Most graduate schools have required a 3.0-grade point average since 1975 (the transition began two decades earlier), but some schools still have 2.75 as their pass standard. Some doctoral programs do not have a formal pass standard, but it is unlikely that they would retain a student who is doing work below 'B' quality. Most American law schools require no more than a 2.0-grade point average to qualify for the professional doctorate in law. This is because law school grades are usually based on a strict bell curve system which typically results in the failure of 10–30% of first-year students. A few law schools require 2.3 or 2.5 for post-doctoral degrees, such as the American LL.M. or S.J.D. degrees. Regular graduate schools have commonly eliminated the D grade because anything below a C is considered failing. Some high schools, to reflect the varying skill required for different level courses and to discourage students from selecting courses that are considered a source of easy 'A's, will give higher numerical grades for difficult courses, often referred to as a weighted GPA. For example, two common conversion systems used in honors and Advanced Placement courses are:  Denver Public Schools uses a different system in honors and AP courses to get weighted GPA values; the scale is as follows:  Another policy commonly used by 4.0-scale schools is to mimic the eleven-point weighted scale (see below) by adding a .33 (one-third of a letter grade) to honors or advanced placement class. (For example, a B in a regular class would be a 3.0, but in honors or AP class it would become a B+, or 3.33).