May, should, and shall. These three words can be found sprinkled across industry standards, recommended practices, and internal company procedures and guidelines. But, what do they really mean?

Most of my work with industry standards happens with the American Petroleum Institute (API), so I will admittedly approach this topic mostly from that perspective. The definitions that follow come from API’s Procedures for Standards Development:

  • May: Denotes a course of action permissible within the limits of a standard.
  • Should: Denotes a recommendation or that which is advised but not required in order to conform to the standard.
  • Shall: Denotes a minimum requirement in order to conform to the standard.

Let’s work through these terms in increasing order of important. The term “may” describes an action that you can take, but there is no requirement that you must do this. For example, API 11B (the standard governing sucker rods) states in section 7.2.2 that calibration intervals, “may be lengthened or shortened based on calibration history.” So, once you establish sufficient calibration history as required by the standard, you can extend the calibration interval if you want to. If instead you prefer to always follow the specific interval written in the standard, you can do that too. May statements leave the choice entirely to the end user if they follow that course of action or not.

“Should” describes something that is strongly recommended but still not a requirement of the standard or procedure in question. The API standard for surface pumping units, API 11E, states in section 6.4.9 that, “the tensile preload in the bolt, stud, or cap screw should be 70 % of the yield strength of the material …”. Now, there are other ways of determining the proper preload on a connection, but the standard is recommending this one possible way to do so. If you do it this way, great! You will be in compliance with the standard. If you would like to specify the connection requirements in a different way, and can demonstrate that method performs as well or better as the should statement in the standard, great! You have the flexibility to apply whichever you believe works best for your product and manufacturing processes.

Now we come to the heavy lifter, the “shall” statement. Shall statements say you absolutely, positively, must follow this statement to comply with the standard or procedure in question. The twenty-seventh edition of API 11B has 544 instances of the word shall spread throughout it! Section covers material test reports, and has some requirements for how long these records must be kept. “These documents shall be maintained by the manufacturer for a period of at least five years.” If someone trashes a pile of MTRs after four years and 364 days, they are not in compliance with the standard.

Let’s relate this back to something more applicable to everyday life. You want your kids to brush their teeth every night before bed, right? Which of the above terms best describe your desire for them to follow this routine? Would you prefer that they may brush their teeth, they should brush their teeth, or they shall brush their teeth?