Ego depletion

Ego depletion refers to the idea that self-control or willpower draws upon a limited pool of mental resources that can be used up.[1] When the energy for mental activity is low, self-control is typically impaired, which would be considered a state of ego depletion. In particular, experiencing a state of ego depletion impairs the ability to control oneself later on. A depleting task requiring self-control can have a hindering effect on a subsequent self-control task, even if the tasks are seemingly unrelated. Self-control plays a valuable role in the functioning of the self on both individualistic and interpersonal levels. Ego depletion is therefore a critical topic in experimental psychology, specifically social psychology, because it is a mechanism that contributes to the understanding of the processes of human self-control. There have both been studies to support[2] and to question[3] the validity of ego-depletion as a theory.

The word "ego" in "ego depletion" is used in the "psychological" sense rather than the colloquial sense.

Some meta analyses and studies have questioned the size and existence of the ego depletion effect.[4] The ultimate validity of those later studies are not universally agreed about.[5]

Early experimental evidence

American social psychologist Roy Baumeister and his colleagues proposed a model that described self-control like a muscle, which can become both strengthened and fatigued. The researches proposed that initial use of the “muscle” of self-control could cause a decrease in strength, or ego depletion, for subsequent tasks. Later experimental findings showed support for this muscle model of self-control and ego depletion.[6]

A key experiment by Baumeister, Ellen Bratslavsky, Mark Muraven and Dianne Tice in 1998, demonstrated some of the first evidence that ego depletion had effects in diverse contexts or situations.[1] They showed that people who initially resisted the temptation of chocolates were subsequently less able to persist on a difficult and frustrating puzzle task. They attributed this effect to ego depletion, which resulted from the prior resisting of a tempting treat. Additionally, it was demonstrated that when people voluntarily gave a speech that included beliefs contrary to their own, they were also less able to persist on the difficult puzzle, indicating a state of ego depletion. This effect was not nearly as strong when individuals were not given a choice and were "forced" to write a counter-attitudinal speech. Thus, they proposed that both the act of choice and counter-attitudinal behaviors draw upon the same pool of limited resources. While giving a counter-attitudinal speech is expected to produce ego depletion, introducing the element of choice further increases the level of experienced depletion. These findings demonstrated the effects of ego depletion in differential situations and emphasized that ego depletion may not be context-specific. This experiment was critical in that the researchers synthesized ideas proposed by prior studies that had suggested evidence for a strength model of willpower. With this study, Baumeister and his colleagues therefore provided the first direct experimental evidence of ego depletion, and initiated research interest on the subject.

Physiological causes

The role of glucose as a specific form of energy needed for self-control has been explored by researchers. Glucose, a sugar found in many foods, is a vital fuel for the body and the brain. Multiple experiments have connected self-control depletion to reduced blood glucose, and suggested that self-control performance could be replenished by consuming glucose (e.g., lemonade).[7] However, some of the findings were later questioned.[8] Several recent experiments have found that resource depletion effects can be reversed by simply tasting (but not swallowing or consuming) sweet beverages,[9][10][11] which can have rewarding properties.[12] Others have suggested that the taste of sugar (but not artificial sweetener) has psycho-physiological signaling effects.

An experiment by Segertrom (2007) and Solberg Nes, has shown that HRV (heart rate variability) is a marker for both ego depletion, and an index for self-control power before the task.[13]

The underlying neural processes associated with self-control failure have been recently examined using neurophysiological techniques. According to cognitive and neuroscientific models of mental control, a "conflict-monitoring/error-detection system" identifies discrepancies between intended goals and actual behaviors.[14] Error-related negativity (ERN) signals are a waveform of event-related potentials, which appear to be generated in the anterior cingulate cortex when individuals commit errors in various psychological tasks.[15] Using electroencephalography (EEG) recordings, Inzlicht and Gutsell found that individuals who had undergone an emotion-suppression task displayed weaker ERN signals compared to individuals who had not undergone emotion-suppression tasks.[14] These findings demonstrate preliminary evidence that depletion experienced after exerting self-control can weaken neural mechanisms responsible for conflict monitoring.

The majority of ego depletion studies have been carried out on university students, which raises concerns about how generalizable the results really are. The effects of age are unknown, but maybe younger people are more susceptible to the effects of ego depletion, given that the areas of the brain involved in self-control continue to develop until the mid 20s. For example, a recent study found that people over the age of 40 did not become ego depleted following a typical depletion manipulation, whereas younger university students did.[16]

Manifestations

Guilt and prosocial behavior

Ego depletion has also been implicated in guilt and prosocial behavior. The feeling of guilt, while unpleasant, is necessary to facilitate adaptive human interactions.[17] The experience of guilt is dependent on one’s ability to reflect on past actions and behaviors. Ego depletion has been shown to hinder the ability to engage in such reflection, thereby making it difficult to experience guilt.[18] Since guilt typically leads to prosocial behavior, ego depletion will therefore reduce the good deeds that often result from a guilty conscience. In the study by Xu and colleagues, some participants were required to suppress their emotions while watching a movie about butchering animals, which resulted in a depleted state.[18] Participants were then induced to feel guilty by playing a game in which an opponent player was blasted with loud, unpleasant noises when they made errors. At the end of the experiment, participants were given a chance to leave money for a subsequent participant and were also given the choice of making a charitable donation. These were the measures of pro-social behavior. The results of this study indicated that people who experienced ego depletion felt less guilty and donated less money than non-depleted people. This demonstrates that ego depletion has an indirect effect on prosocial behavior by decreasing one’s ability to experience guilt.

Perceived levels of fatigue

An individual’s perceived level of fatigue has been shown to influence their subsequent performance on a task requiring self-regulation, independent of their actual state of depletion.[19] This effect is known as illusory fatigue. This was shown in an experiment in which participants engaged in a task that was either depleting or non-depleting, which determined each individual’s true state of depletion. Ultimately, when participants were led to believe their level of depletion was lower than their true state of depletion, they performed much better on a difficult working memory task. This indicates that an increased perceived level of fatigue can hinder self-regulatory performance independent of the actual state of depletion.

Motivation and beliefs

Ego depletion has been shown to have some rather debilitating consequences, most notably self-regulation impairments. These effects can, however, be temporarily buffered by external motivations and beliefs in unlimited willpower. An example of such an external motivator was demonstrated by Boucher and Kofos in 2012, where depleted participants who were reminded of money performed better on a subsequent self-control task.[20]

An experiment by Carol Dweck and subsequent work by Roy Baumeister and Kathleen Vohs has shown that beliefs in unlimited self-control helps mitigate ego depletion for a short while, but not for long. Participants that were led to believe that they will not get fatigued performed well on a second task but were fully depleted on a third task.[21]

Real-life implications

In a state of ego depletion, an individual’s impaired ability to self-regulate can be implicated in a wide range of undesirable and maladaptive behaviors, such as acts of aggression. Knowledge and strategies to counteract ego depletion would therefore, be highly beneficial in various real-life situations.

Dieting

An experiment performed by Kathleen Vohs and Todd Heatherton demonstrated how ego depletion is particularly relevant when considering chronic dieters compared to non-dieters.[22] Chronic dieters constantly work at resisting their cravings and limiting their food intake. Vohs and Heatherton showed that the task of regulating food intake could be undermined in the face of tempting snacks, especially when the individual was experiencing a state of ego depletion. Both dieters and non-dieters attempted to suppress their emotional responses while watching a movie. Afterwards, participants were required to consume ice cream in order to engage in a taste-test. The major finding was that dieters who suppressed their emotional responses to the movie experienced more ego depletion than those who were not required to suppress their emotions. Additionally, those individuals subsequently ate much more ice cream in the taste-testing task. Non-dieters did not show the same self-regulatory failures as dieters in these tasks. Therefore, it seems that the act of dieting itself is a form of resource expenditure. Dieters spend so much energy trying to limit their food intake, but these efforts are likely to be undermined when faced with overwhelming temptation.

Athletic performance

Research has found that competitive athletes’ mental determination can be hindered after completing a difficult cognitive task more than after completing an easy cognitive task.[23] This indicates that the hindering effects of ego depletion can be applied not only to subsequent performance on cognitive tasks, but on physical tasks, as well.

Consumer behavior

In the world of consumerism, individuals are faced with decisions and choices that require the use of valuable energy resources in order to make informed purchases while resisting temptation of impulsive or unnecessary purchases. Consumers are constantly bombarded with a broad range of options. In order to make the best choice, one must compare the many different aspects of various products. The complexity of consumer decisions in itself can result in ego depletion.[24] This, in turn, could impact any subsequent decisions consumers must make. When consumers are depleted, they are more likely to become passive, and make more impulsive decisions that may not fall in line with their true values.

Consumers are faced with choices of different price ranges and product qualities in the market. Having many options can make consumers feel overwhelmed, causing ego depletion.[25] With advertisement telling consumers how they deserve and must have a product. This causes mental fatigue and frustration, leading people to give in to buying a product.[26] Fatigue and frustration can also stem from deals with specific requirements on ways to purchase a product, along with spending effort on deciding which store has the best deals or trying to get to the store. People will then be led to buy the high priced or cheapest product.[27]

Consumers who have low self-control are susceptible to be more invested in obtaining product of a high status.[28] These same consumers are more likely to be more motivated, persistent and pay more for a product. This will lead consumers to have a sense of empowerment; they will feel in control again and feel as though they are overcoming their ego depleted states. It can also lead them to purchasing a brand that has a high status. The consumer might feel that the brand be more beneficial and secure with the product.

Relief

In a recent experiment, it was shown that inducing a positive mood can buffer the impairing effects of ego depletion on subsequent performance.[29] Positive mood was induced by getting individuals to watch comedy videos or by giving them a surprise gift. Positive mood seemed to allow people to recover faster from ego depletion and furthermore, improved their ability to self-regulate. There is no claim that positive mood can provide a general benefit to people who had not previously engaged in self-regulatory tasks; rather, positive mood can restore depleted individuals’ capacity to self-regulate. Furthermore, this experimental work does not consider in depth the mechanisms by which performance is restored. It is not known whether positive mood counteracts ego depletion or whether positive mood merely motivates an individual to persist in a task, despite their depleted state.

The ego depletion effect itself (without mood intervention), has however been shown to be unrelated to mood changes, as shown in multiple ego depletion experiments that either controlled for mood, or saw no mood changes. Thus, positive affect is just a way to counteract ego depletion after a person is depleted.

Theoretical explanations

Conservation hypothesis

The conservation hypothesis is a partial explanation of ego depletion. It suggests that there are two sorts of depletion:

  1. When one is completely depleted and unable to self-control.
  2. When one is not fully depleted, but partly. Still, one reduces his self-control efforts to avoid complete exhaustion.[30]

According to this view, when people feel depleted, there might still exist a reserve store of energy to be used in extreme, high priority situations that could be encountered in the future. This can be adaptive to the extent that expending any more resources at a given time might render an individual fully depleted of their resources in an unexpected situation requiring self-regulation or other self-monitoring behaviours. The existence of a spare reservoir of mental energy ultimately explains why various motivators can buffer the effects of mild or moderate ego depletion. In a state of low resources, an individual lacks motivation to exert any more energy, but if motivation is presented, there are still extra resources that can be used up. Thus, ego depletion could be conceptualized as a psychological constraint necessary to safeguard precious resources that might be needed in emergency situations in the future. Under mild depletion, people still have a small amount of energy left in their "tank", which they do not have access to under normal circumstances.[30]

Criticism

Questions and alternative explanations

Although self-control has traditionally been thought of as a limited resource that can be depleted, some researchers disagree with this model. While multiple studies provided support for the ego depletion effect, there is currently no direct measure of ego depletion, and studies mainly observe it by measuring how long people persist at a second task after performing a self-control task (the depleting task).[1] The theory of ego depletion relies on the inner workings of an individual's volition, which can only be indirectly tested; therefore, only inferences can be made. Another challenge facing research on ego depletion is the influence of the overall mental conditions of individuals being studied. There is speculation that results may be disrupted in individuals who report experiencing depression and already possess high levels of ego depletion prior to the study.

Many ego depletion studies, however, have shown that mood is not relevant to the results. In fact, many of the earlier experiments have tested for the effects of mood and saw no effect of mood whatsoever. Furthermore, the study and measurement of ego depletion may be affected by the confounding effect of cognitive dissonance. Researchers have questioned whether subjects are truly experiencing ego depletion, or whether the individuals are merely experiencing cognitive dissonance in the psychological tasks.[1]

Process model

In contrast to the original most known model of self-control, Michael Inzlicht and Brandon J. Schmeichel propose an alternative model of depletion, which they refer to as the process model.[31] This process model holds that initial exertions of willpower lead an individual's motivation to shift away from control, and towards gratification. As a part of this process, one's attention shifts away from cues that signal the need for control, and towards cues that signal indulgence. Inzlicht and Schmeichel argue that the process model provides a starting point for understanding self-control and that more research examining these cognitive, motivational, and affective influences on self-control is needed.

Reproducibility controversy and conflicting meta analyses

In 2016, a major study (2141 participants) carried out at two dozen labs across the world using a single protocol failed to find any evidence for ego depletion.[4][32] In response, Baumeister argued that his original protocol was rejected by the project coordinators, and after discussion was stalled, he only reluctantly agreed to a task that differed to some degree from the original 1998 studies.[33] Replication difficulties have also emerged for 5 additional protocols (operationalizations) of the basic ego depletion effect.[34]

A 2010 meta analysis of 198 independent tests found the effect significant with a moderate to large average effect size (d = .6). Even after accounting for possible unpublished failed studies, the analysis concluded that it is extremely unlikely that the effect is untrue.[35]

In 2015, a meta analysis of over 100 studies by Carter and McCullough argued that the 2010 meta-analysis failed to take publication bias into account. They showed statistical evidence for publication bias. When they statistically controlled for publication bias, the effect size estimate was small (d = .2) and not significantly different from zero.[36][37] Michael Inzlicht and colleagues praised Carter's meta analysis, but argued that bias-correction techniques are not precise enough to give a precise control size estimate.[5][38] In response, Cunningham and Baumeister argued that Carter and McCullough analysis contained errors in its data collection and in the various analyses used.[5]

Ulrich Schimmack conducted a meta-analysis of published studies and found that most studies could produce significant results only with the help of random sampling error. Based on the low power of studies, one would expect a large number of non-significant results, but these results are missing from published articles. This finding confirms Carter and McCullough's meta-analysis that showed publication bias with a different statistical method. Schimmack's replicability report also identified a small set of studies with adequate power that provided evidence for ego-depletion. These studies are the most promising studies for a replication project to examine whether ego-depletion effects can be replicated consistently across several independent laboratories.[39]

See also

References

  1. 1 2 3 4 Baumeister, R. F.; Bratslavsky, E.; Muraven, M.; Tice, D. M. (1998). "Ego depletion: Is the active self a limited resource?" (PDF). Journal of Personality and Social Psychology. 74 (5): 1252–1265. PMID 9599441. doi:10.1037/0022-3514.74.5.1252.
  2. ↑ Baumeister, Roy F. (2002-04-01). "Ego Depletion and Self-Control Failure: An Energy Model of the Self's Executive Function". Self and Identity. 1 (2): 129–136. ISSN 1529-8868. doi:10.1080/152988602317319302.
  3. ↑ Engber, Daniel (2016-03-06). "Everything Is Crumbling". Slate. ISSN 1091-2339. Retrieved 2017-01-11.
  4. 1 2 Engber, Daniel (2016-03-06). "Everything Is Crumbling". Slate. ISSN 1091-2339. Retrieved 2016-03-07.
  5. 1 2 3 Cunningham, MR; Baumeister, RF. "How to Make Nothing Out of Something: Analyses of the Impact of Study Sampling and Statistical Interpretation in Misleading Meta-Analytic Conclusions". Frontiers in Psychology. 7: 1639. PMC 5079083 Freely accessible. PMID 27826272. doi:10.3389/fpsyg.2016.01639.
  6. ↑ Baumeister, Roy F. (2002). "Ego Depletion and Self-Control Failure: An Energy Model of the Self's Executive Function". Self and Identity. 1 (2): 129–136. doi:10.1080/152988602317319302.
  7. ↑ Gailliot, M. T.; Baumeister, R. F.; Dewall, C. N.; Maner, J. K.; Plant, E. A.; Tice, D. M.; Brewer, B. J.; Schmeichel, Brandon J. (2007). "Self-control relies on glucose as a limited energy source: Willpower is more than a metaphor". Journal of Personality and Social Psychology. 92 (2): 325–336. PMID 17279852. doi:10.1037/0022-3514.92.2.325.
  8. ↑ Kurzban, R. (2010). "Does the brain consume additional glucose during self-control tasks?". Evolutionary Psychology. 8: 244. doi:10.1177/147470491000800208.
  9. ↑ Hagger, M.S.; Chatzisarantis, N.L. (2013). "The Sweet Taste of Success The Presence of Glucose in the Oral Cavity Moderates the Depletion of Self-Control Resources". Personality and Social Psychology Bulletin. 39: 28–42. doi:10.1177/0146167212459912.
  10. ↑ Molden, D.C.; Hui, C.M.; Scholer, A.A.; Meier, B.P.; Noreen, E.E.; d'Agostino, P.R.; Martin, V. (2012). "Motivational versus metabolic effects of carbohydrates on self-control". Psychological Science. 23 (10): 1137–1144. doi:10.1177/0956797612439069.
  11. ↑ Sanders, M.A.; Shirk, S.D.; Burgin, C.J.; Martin, L.L. (2012). "The Gargle Effect Rinsing the Mouth With Glucose Enhances Self-Control". Psychological Science. 23 (12): 1470–1472. doi:10.1177/0956797612450034.
  12. ↑ Frank, G.K.; Oberndorfer, T.A.; Simmons, A.N.; Paulus, M.P.; Fudge, J.L.; Yang, T.T.; Kaye, W.H. (2008). "Sucrose activates human taste pathways differently from artificial sweetener". NeuroImage. 39 (4): 1559–1569. doi:10.1016/j.neuroimage.2007.10.061.
  13. ↑ Segerstrom SC, Nes LS (2007). "Heart rate variability reflects self-regulatory strength, effort, and fatigue". Psychol Sci. 18 (3): 275–81. PMID 17444926. doi:10.1111/j.1467-9280.2007.01888.x.
  14. 1 2 Inzlicht, M.; Gutsell, J. N. (2007). "Running on empty: Neural signals for self-control failure". Psychological Science. 18 (11): 933–937. PMID 17958704. doi:10.1111/j.1467-9280.2007.02004.x.
  15. ↑ Holroyd, C. B.; Coles, M. G. H. (2002). "The Neural Basis of Human Error Processing: Reinforcement Learning, Dopamine, and the Error-related Negativity". Psychological Review. 209 (4): 679–709. doi:10.1037/0033-295X.109.4.679.
  16. ↑ Dahm, T; Neshat-Doost, HT; Golden, A-M; Horn, E; Hagger, M; et al. (2011). "Age Shall Not Weary Us: Deleterious Effects of Self-Regulation Depletion Are Specific to Younger Adults". PLoS ONE. 6 (10): e26351. doi:10.1371/journal.pone.0026351.
  17. ↑ Baumeister, R. F.; Stillwell, A. M.; Heatherton, T. F. (1994). "Guilt: An interpersonal approach". Psychological Bulletin. 115 (2): 243–267. PMID 8165271. doi:10.1037/0033-2909.115.2.243.
  18. 1 2 Xu, H.; Bègue, L.; Bushman, B. J. (2012). "Too fatigued to care: Ego depletion, guilt, and prosocial behavior". Journal of Experimental Social Psychology. 43 (5): 379–384. doi:10.1016/j.jesp.2012.03.007.
  19. ↑ Clarkson, J. J.; Hirt, E. R.; Austin Chapman, A. D.; Jia, L. (2010). "The impact of illusory fatigue on executive control: Do perceptions of depletion impair working memory capacity?". Social Psychological and Personality Science. 2 (3): 231–238. doi:10.1177/1948550610386628.
  20. ↑ Boucher, H. C.; Kofos, M. N. (2012). "The idea of money counteracts ego depletion effects". Journal of Experimental Social Psychology. 48 (4): 804–810. doi:10.1016/j.jesp.2012.02.003.
  21. ↑ Vohs, K. D.; Baumeister, R. F.; Schmeichel, B. J. (2012). "Motivation, personal beliefs, and limited resources all contribute to self-control". Journal of Experimental Social Psychology. 48 (4): 943–947. doi:10.1016/j.jesp.2012.03.002.
  22. ↑ Vohs, K. D.; Heatherton, T. F. (2000). "Self-regulatory failure: A resource-depletion approach". Psychological Science. 11 (3): 249–254. PMID 11273412. doi:10.1111/1467-9280.00250.
  23. ↑ Dorris, D. C.; Power, D. A.; Kenefick, E. (2011). "Investigating the effects of ego depletion on physical exercise routines of athletes". Psychology of Sport and Exercise. 13 (2): 118–125. doi:10.1016/j.psychsport.2011.10.004.
  24. ↑ Baumeister, R. F.; Sparks, E. A.; Stillman, T. F.; Vohs, K. D. (2008). "Free will in consumer behavior: Self-control, ego depletion, and choice". Journal of Consumer Psychology. 18: 4–13. doi:10.1016/j.jcps.2007.10.002.
  25. ↑ Hofmann, W.; Strack, F.; Deutsch, R. (2008). "Free to buy? Explaining self-control and impulse in consumer behavior". Journal of Consumer Psychology. 18 (1): 22–26. doi:10.1016/j.jcps.2007.10.005.
  26. ↑ Schmeichel, B.J.; Vohs, K.D.; Baumeister, R.F. (2003). "Intellectual performance and ego depletion: Role of the self in logical reasoning and other information processing". Journal of Personality and Social Psychology. 85: 33–46. doi:10.1037/0022-3514.85.1.33.
  27. ↑ Baumeister, R. F.; Sparks, E. A.; Stillman, T. F.; Vohs, K. D. (2008). "Free will in consumer behavior: Self-control, ego depletion, and choice". Journal of Consumer Psychology. 18 (1): 4–13. doi:10.1016/j.jcps.2007.10.002.
  28. ↑ CUTRIGHT, K. M.; SAMPER, A. (2014). "Doing It the Hard Way: How Low Control Drives Preferences for High-Effort Products and Services". Journal of Consumer Research. 41 (3): 730–745. doi:10.1086/677314.
  29. ↑ Tice, D. M.; Baumeister, R. F.; Shmueli, D.; Muraven, M. (2007). "Restoring the self: Positive affect helps improve self-regulation following ego depletion". Journal of Experimental Social Psychology. 43 (3): 379–384. doi:10.1016/j.jesp.2006.05.007.
  30. 1 2 Baumeister, R. F.; Vohs, K. D. (2007). "Self-regulation, ego depletion, and motivation". Social and Personality Psychology Compass. 1: 115–128. doi:10.1111/j.1751-9004.2007.00001.x.
  31. ↑ Inzlicht, M.; Schmeichel, B. J. (2012). "What is ego depletion? Toward a mechanistic revision of the resource model of self-control". Perspectives on Psychological Science. 7 (5): 450–463. doi:10.1177/1745691612454134.
  32. ↑ "RRR – The Ego-Depletion Paradigm - Association for Psychological Science". www.psychologicalscience.org. Retrieved 2016-04-20.
  33. ↑ "Misguided Effort with Elusive Implications - Association for Psychological Science" (PDF). www.psychologicalscience.org. Retrieved 2016-04-20.
  34. ↑ "Curated independent direct replications of ego depletion effect". www.curatescience.org. Retrieved 2016-11-26.
  35. ↑ Hagger, MS; Wood, C; Stiff, C; Chatzisarantis, NL (July 2010). "Ego Depletion and the Strength Model of Self-Control: A Meta-Analysis" (PDF). Psychological Bulletin. 136: 495–525. PMID 20565167. doi:10.1037/a0019486.
  36. ↑ "Supplemental Material for A Series of Meta-Analytic Tests of the Depletion Effect: Self-Control Does Not Seem to Rely on a Limited Resource". Journal of Experimental Psychology: General. doi:10.1037/xge0000083.supp.
  37. ↑ Carter, Evan C.; McCullough, Michael E. (2014-01-01). "Publication bias and the limited strength model of self-control: has the evidence for ego depletion been overestimated?". Personality and Social Psychology. 5: 823. PMC 4115664 Freely accessible. PMID 25126083. doi:10.3389/fpsyg.2014.00823.
  38. ↑ Inzlicht, Michael; Gervais, Will; Berkman, Elliot (2015-09-11). "Bias-Correction Techniques Alone Cannot Determine Whether Ego Depletion is Different from Zero: Commentary on Carter, Kofler, Forster, & McCullough, 2015". Rochester, NY: Social Science Research Network. SSRN 2659409 Freely accessible.
  39. ↑ R, Dr (2016-04-18). "Replicability Report No. 1: Is Ego-Depletion a Replicable Effect?". Replicability-Index. Retrieved 2017-01-12.

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