I pulled this off Wikipedia as a reference and it has some interesting definitions, links and empirical data.
If you DISC or personality test your candidates, perhaps a "Grit" test would be a better gauge of future success, especially in sales.
Grit in psychology is a positive, non-cognitive trait, based on an individual’s passion for a particular long-term goal or endstate coupled with a powerful motivation to achieve their respective objective. This perseverance of effort promotes the overcoming of obstacles or challenges that lie within a gritty individual’s path to accomplishment and serves as a driving force in achievement realization. Commonly associated concepts within the field of psychology include "perseverance," "hardiness," "resilience,” “ambition,” “need for achievement” and conscientiousness. These constructs can be conceptualized as individual differences related to the accomplishment of work rather than latent ability.This distinction was brought into focus in 1907 when William James challenged the field to further investigate how certain individuals are capable of accessing richer trait reservoirs enabling them to accomplish more than the average person, but the construct dates back at least to Galton, and the ideals of persistence and tenacity have been understood as a virtue at least since Aristotle. Although the last decade has seen a noticeable increase in research focused on achievement-oriented traits, there continues to be difficulty in aligning specific traits and outcomes.
Definition of Grit
Grit is defined as “perseverance and passion for long-term goals.”  Building upon biographical collections of famous leaders in history, researchers and scientists have reached similar conclusions about high achieving individuals. Specifically, those individuals who were deemed more successful and influential than their contemporary counterparts typically possessed traits above and beyond that of normal ability. While ability was still critically important, these individuals also possessed “zeal” and “persistence of motive and effort.” Duckworth and colleagues (2007) believe this dual-component of Grit to be a crucial differentiator from similar constructs. Grit is conceptualized as a stable trait that does not require immediate positive feedback. Individuals high in Grit are able to maintain their determination and motivation over long periods of time despite experiences with failure and adversity. Their passion and commitment towards the long-term objective is the overriding factor that provides the stamina required to “stay the course” amid challenges and set-backs. Essentially, the Grittier person is focused on winning the marathon, not the sprint.
Grit and Positive Psychology
Grit also ties in with positive psychology and in particular, with perseverance. As mentioned earlier, the ability to stick with and pursue a goal over a long period of time is an important aspect of Grit. This area of positive psychology has been interested in the process of perseverance as a positive indicator of long term success. Grit’s inclusion of the perseverance construct is perhaps unsurprising as Angela Duckworth was a doctoral candidate under Martin Seligman.
Grit and Intelligence
One of the best predictors of future achievement has been intelligence. This relationship has been found in scholastic achievement as well as in job performance. As such, one might expect that grit would be strongly correlated with intelligence. In fact, this prompted one of the early questions asked in Grit research, “Why do some individuals accomplish more than others of equal intelligence?”. Somewhat surprisingly, in four separate samples, Grit was found to be either orthogonal to or slightly inversely correlated with intelligence. This means that Grit, unlike many traditional measures of performance is not tied to intelligence. As the researchers have suggested, this helps explain why some very intelligent individuals do not consistently perform well over long periods of time.
 Grit and Personality Measures
The Grit measure has been compared to the Big Five personality model, which are a group of broad personality dimensions consisting of openness to experience, conscientiousness, extraversion, agreeableness, and neuroticism. In one study by Duckworth and Quinn, the Short Grit Scale (Grit–S) and 12-item self-report measure of Grit (Grit–O) measuring Grit was strongly correlated with conscientiousness (r = .77, p <.001 and r = .73, p <.001) (2009). While Grit is related to conscientiousness measures, it also differs from conscientiousness in important ways. For example, while both Grit and conscientiousness are often associated with short term accomplishments, Grit is also associated with longer term and multi-year goals. This long-term persistence and dependability are important aspects that make Grit distinct from conscientiousness. Another personality characteristic that is often linked to Grit is the need for achievement. One way in which Grit differs from the need for achievement is that individuals with high scores in Grit often set extremely long-term goals for themselves and pursue them deliberately even without positive feedback, while need for achievement lacks this long-term component.
Traditional constructs in this area include perseverance, hardiness, resilience, ambition, and need for achievement. Grit has been argued to be distinguishable from each of these in the following ways. Perseverance is the steadfast pursuit of a task, mission, or journey in spite of obstacles, discouragement, or distraction. In contrast, Grit is argued to be trait to perseverance. Grit enables an individual to persevere in accomplishing a goal despite obstacles over an extended period of time. When compared with the construct of persistence, Grit adds a component of passion for the goal. This goal passion also contributes to the ability of the individual to sustain effort over the long term.
Maddi (2006) defines hardiness as a combination of attitudes that provide the courage and motivation to do the hard, strategic work of turning stressful circumstances from potential disasters into growth opportunities. While Grit is primarily a measure of an individual’s ability to persist in obtaining a specific goal over an extended time period (Duckworth et al., 2007), hardiness refers to an individual’s ability to persist through difficult circumstances and does not address the individual’s long term persistence toward a specific goal. Maddi (2006) developed a theoretical model of hardiness as a tool for developing resilience.
Resilience is a dynamic process in which an individual overcomes significant adversity, usually in the form of a life changing event or difficult personal circumstances. Resilience can be conceptualized as an adaptive response to a challenging situation. Grit involves maintaining goal focused effort for extended periods of time, often while facing adversity but does not require a critical incident. Importantly, Grit is conceptualized as a trait while resilience is a dynamic process. Finally, resilience has been almost exclusively studied in children (cf. Luther, Doernberger, & Zigler, 1993) who are born into “at-risk” situations. Although resilience researchers recognize that adults likely demonstrate resilience in a similar manner to children, the resilience process has not been studied in a mature population.
Ambition is broadly defined as the desire for attainment, power, or superiority. In contrast to ambitious individuals, Gritty individuals do not seek fame or external recognition for their achievements. Ambition is often associated with a desire for fame. Unlike ambitious individuals, gritty individuals do not seek to distinguish themselves from other people, but to obtain personal goals.
McClelland (1961) describes need for achievement as a drive to complete manageable goals that enable the individual to receive immediate feedback. In contrast to need for achievement, Gritty individuals consciously set long-term goals that are difficult to attain and do not waver from these difficult goals, regardless of the presence of feedback. Additionally, need for achievement has been studied for almost 50 years and has been found to positively correlate to self-efficacy and learning goal orientation. These links have not yet been tested in the Grit literature.
The primary scientific findings on Grit come from Duckworth and colleagues’ examination of Grit as an individual difference trait capable of predicting long-term success. It was proposed that individuals who possess a drive to tirelessly work through challenges, failures, and adversity to achieve set goals and are uniquely positioned to reach higher achievements than others who lack similar stamina. In a series of six studies Duckworth et al. proposed, developed, and tested a two-factor Grit scale with notable results. In addition to validating their Grit scale, the authors also found support suggesting that Grit provided incremental predictive validity for education and age above and beyond the Big 5 personality traits (Study 2); that higher levels of Grit were more highly associated with cumulative grade point average (GPA) in an Ivy league sample when compared to those with lower Grit levels (r = .25, p < .01; Study 3); that Grit predicted retention after their first summer in two classes of cadets at the United States Military Academy (Study 4); and that participants in a National Spelling Bee with higher Grit scores typically work harder and longer than less Gritty peers, ultimately resulting in better performance. This series of studies provides empirical evidence that an individual difference conceptualized as Grit can account for significant variance in performance across a variety of settings. Grit predicts beyond the typical and unrelated cognitive construct of IQ and can account for variance over and above what is observed in the Big 5 personality construct of conscientiousness.In 2009, Duckworth and Quinn found additional support for the Grit construct when they developed and validated a more condensed version of the Grit Scale (Grit-S) by removing four of the previous items and improving its psychometric properties. Using samples from the data collected in their 2007 studies, the authors were able to achieve complementary results that suggested the positive relationships between Grit and educational attainment, GPA, retention in college, and success in a national spelling bee competition.
Questions may surface relating to what additional cognitive and non-cognitive traits play complementary roles in the development of Grit. Of additional interest may be how the distinctive environmental conditions, specifically the interrelationships of emotional and cognitive load, might moderate and assist in explaining why some individuals succumb to significant challenges or struggle with obstacles that block their path to goal achievement, while others are able to overcome these barriers. The United States military believes that this and similar constructs may assist in explaining why some soldiers are better equipped to handle the psychological trauma of combat. Other on-going work includes investigations of the combined or multiplicative impacts of both cognitive and Grit-like predictors of achievement in leader adaptability situations.