The question of when Master numbers should be reduced to single-digits in Numerology, and when they should be left in their double-digit state is one I get often. To answer that question, we have to know when Master numbers affect us as Master numbers, and when they do not. If Master numbers (11, 22 and 33) are found in places where they can actually deliver their "masterly" potential, they should not be further reduced. If they are found in a location where they do not have that power we reduce them to single digits. It's all location, location, location.
For example, when we calculate a Personal Year number and the result is a Master number, we reduce it to a single digit because a Personal Year cycle does not have the power to influence us to the potential of a Master number. So that is the basic bench mark; can a particular cycle ,or trait live up to Master number potential? If not, reduce it to a single digit. Personal Year, Personal Month and Personal Day cycles cannot. Neither can Universal cycles, therefore they should always be reduced to single digits. Almost all numbers that are part of your ,personality chart do have that potential, and Master numbers found there are not reduced. Essence cycles, Period cycles, and Pinnacle cycles also have the kind of power that warrants not reducing ,Master numbers.
In many places, especially when it comes to compatibility charts, there is a bit of a gray area due to the ambiguous nature of Master numbers. If you have a relationship with someone who has a Master number, you can reduce that to a single digit or leave it a Master number, the difference is negligible. However, a Master number tends to make one more vulnerable to phobias and eccentric behavior, usually in line with the traits associated to the single digit value of that Master number. For example, an 11 Expression has all the behavioral traits of a 2 within a relationship environment (sensitive, vulnerable, feminine, non-confrontational, tactful, manipulative, considerate, demanding and so forth), but add to that a dose of irrational sensitivity, nervous tension and selfishness brought by the Master number 11 to understand how the 11's involvement can affect the relationship as well. In the same light, a 22 Expression will have all the traits of the 4 when viewed as a relationship compatibility number (solid, reliable, trustworthy, down-to-earth, dependable, predictable, boring, etc.), but a different set of quirky traits might be affecting the relationship thanks to the 22, such as obsessive-compulsive behavior, fear of open spaces, and a dependence on routine habits and precise time lines that, when broken, wreak havoc on that individual's peace of mind. And lastly, a 33 Expression has all the qualities of the 6 (loving, caring, responsible, self-sacrificing, intrusive, gossipy, etc.), but throws in a mixture of arrogance and condescending attitude, that can be irritating, to say the least.
As you can see, Master numbers are not always all they are cracked up to be. If a chart is strong and supportive, they have the ability to make great things happen. If the chart does not show the kind of effort and focus required to take advantage of Master numbers, they can be a burden. Perhaps the most telling clue in a chart, as far as any possible Master number potential, is balance and harmony; a fairly even spread of all numbers without too many contradictions. That is the kind of chart usually found to deliver a level of effort that spells success. After all, in the end there is only one quality that decides an individual's achievements: effort. (There is a story supposedly told by the late Russian master Gurdjev that there are three kinds of people: those that put forth no effort, those that do, and those that deliver extra effort. Those that put forth no effort achieve nothing and tend to live a life of poverty and dissatisfaction. Those that do have the potential to live a "normal" life of some success, marriage, kids, and perhaps a house with a picket fence. It is those who deliver the extra effort, who always go the extra mile, cut no corners, and demand the utmost from themselves, who make great things happen. Oh well, that's just Gurdjev for you.)