CHRONIC PAIN after spinal cord injury (SCI) can be a

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1 1571 Chronic Pain After Spinal Injury: Interference With Sleep and Daily Activities Eva G. Widerström-Noga, DDS, PhD, Ernesto Felipe-Cuervo, MS, Robert P. Yezierski, PhD ABSTRACT. Widerström-Noga EG, Felipe-Cuervo E, Yezierski RP. Chronic pain after spinal injury: interference with sleep and daily activities. Arch Phys Med Rehabil 2001; 82: Objectives: To determine how chronic pain after spinal cord injury (SCI) interfered with sleep, exercise, work, household chores, and other daily activities and to define which clinical aspects of pain and psychosocial factors best predicted the extent of interference. Design: Postal survey; follow-up to a previous survey conducted 6 months earlier. Setting: General community. Participants: Individuals (n 217) with traumatic SCI and chronic pain. Intervention: Subjects answered questions regarding frequency of interference caused by pain on 5 activities: sleep, work, exercise, household chores, and other daily activities. Asked to self-report sadness, fatigue, or anxiety; and to describe location, quality, and intensity of pain. Main Outcome Measures: Demographic data (gender, age, level of injury); sociodemographic data (education, employment); self-reported psychosocial outlook; clinical characteristics of pain: location (drawing), quality (descriptors), and intensity (2 numeric rating scales). Regression analysis. Results: The questionnaire was returned by 65.8% of the sample (217/330). A large number of the participants (77.3%) reported frequent interference caused by pain, ie, often to always in 1 or more of the 5 activities. The combination of high pain intensity and the use of multiple pain descriptors was significantly associated with frequent interference with falling asleep. Frequent sleep interruption was significantly associated with high pain intensity, male gender, anxiety, and higher age at time of injury. In working individuals, frequent interference due to pain was significantly associated with multiple pain descriptors, anxiety, low level of education, and being older at time of injury. Conclusion: Reported extent of pain interference in various areas of activity is related to clinical symptoms of pain as well as to psychologic and psychosocial factors rather than level of injury. The relationship between frequent interference, pain intensity, and multiple descriptors indicate that individuals experiencing several types of pain of high intensity are more From the Miami Project to Cure Paralysis (Widerström-Noga, Felipe-Cuervo, Yezierski) and Department of Neurological Surgery, University of Miami (Widerström-Noga, Yezierski), Miami, FL. Accepted in revised form December 8, Supported by the State of Florida, The Miami Project, the Gordon Family Foundation, and the Hollfelder Foundation. No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the author(s) or upon any organization with which the author(s) is/are associated. Reprint requests to Eva G. Widerström-Noga, DDS, PhD, Dept of Neurological Surgery, 1600 NW 10th Ave, R-48, Miami, FL 33136, ewiderst@ miamiproject.med.miami.edu /01/ $35.00/0 doi: /apmr likely than others to experience frequent interference with a variety of daily activities including sleep. Key Words: Activities of daily living; ; Pain, intractable; Pain measurement; Rehabilitation; Spinal cord injuries by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation CHRONIC PAIN after spinal cord injury (SCI) can be a life-long experience that may become progressively more difficult to deal with as individuals age, 1,2 especially because some of these pain conditions tend to be severe. 3-6 In addition, pain types like central neuropathic pain are particularly refractory to treatment 7 and pain management is complicated by an incomplete understanding of which factors cause and sustain these pain conditions. Aside from neuropathic types of pain, there are also several nociceptive types, eg, musculoskeletal pain in the neck and shoulder area, 8-11 and the latter type of pain has been reported to be common in persons with tetraplegia 12 and aggravated by wheelchair transfer. 11,12 The interference in a person s life caused by SCI is extensive, and hence an individual s ability to adapt or adjust to new situations is critical for quality of life (QOL). 13 Psychologic factors such as anxiety, sadness, and excessive fatigue have been reported by individuals experiencing SCI-related pain and shown to affect significantly coping and adjustment to injury. 13,18,19 Chronic pain after SCI pain can cause interference with daily activities, which are over and above the limitations induced by other consequences of injury. 10 This interference may partly explain the relationship between chronic pain after SCI and decreased QOL Specific characteristics of pain, such as degree of muscle tenderness, have previously been shown to be associated with more interference in a person s life. 23 For example, in the dysfunctional cluster of the Multidimensional Pain Inventory 24 (MPI), high levels of pain severity are associated with high levels of interference. Because high levels of interference are likely to result in decreased QOL, an important aspect of pain following SCI is the extent to which that pain interferes with the daily routine of a person s life. Furthermore, because of the complexity of interactions between the various consequences of SCI, 25 it is essential to define relationships between interference in various areas of activity and clinical characteristics of pain as well as psychosocial factors. According to the clusters of the MPI, 26 a person whose pain interferes with a number of activities is likely to experience affective distress. For example, depression can reduce motivation to participate in activities essential for reaching optimal levels functional independence after SCI. 27 The biopsychosocial view on pain incorporates a dynamic interaction between physical, psychologic, and social factors in a time perspective. This perspective not only includes the physical cause of pain but also the psychologic and social factors of importance for maintaining a chronic pain condition. 28 This model provides a theoretical foundation that can guide therapeutic intervention while defining the factors re-

2 1572 CHRONIC PAIN AFTER SCI: LIFE INTERFERENCE, Widerström-Noga sponsible at a particular point in time for a specific pain condition in each individual patient. SCI imposes limitations on an individual s ability to perform and participate in daily activities, and the relationship between pain and decreased QOL 29 highlights the importance of determining the extent to which chronic pain interferes after SCI. Furthermore, it is important to define the factors determining the extent of pain interference in various daily activities because such relationships provide insights for treatment and rehabilitation strategies. Multivariate relationships including frequency of pain interference, clinical characteristics of SCI pain, and psychosocial factors have, to our knowledge, not been previously explored. Our study sought to determine how frequent chronic pain after SCI interfered with daily activities such as sleep, exercise, work, household chores, and to define the clinical aspects of pain and psychosocial factors predicting frequency of interference of these activities. METHODS Subjects In a previous study, persons with traumatic SCI who were 18 years of age or older were recruited from The Miami Project database. Only persons on whom information concerning date of birth, gender, cause of injury, and date of injury were available were included. In that study, 330 reported chronic pain, and of those, 217 (65.8%) subjects, comprising the sample of the present study, volunteered to fill out a detailed pain history and related information. The study was approved by the University of Miami Institutional Review Board. Survey Instrument The present study was a follow-up to a previous survey, conducted 6 months earlier 25 in which 430 subjects rated how hard they found it to deal with various consequences of SCI including pain, on a numeric rating scale (NRS) ranging from 0 (not hard at all) to 10 (extremely hard). In the present survey, the participants filled out a set of questions concerning various clinical characteristics and related factors of chronic pain experienced for at least the last 3 months before the study. To increase the response rate, all individuals not responding within 2 months received a reminder and second copy of the survey. 30 We focused on questions concerning frequency of interference caused by pain on sleep, work, exercise, household chores, and activities of daily living, as well as the relationships between interference and psycho-socio-demographic and clinical characteristics of pain after SCI. Some of the pain characteristics were described previously 6 and are briefly described later. Sociodemographic Data and Characteristics of Injury Demographic information and injury characteristics obtained from The Miami Project database were: age at time of the study, age at injury, and time postinjury (described in years and fractions of years), gender, and level of injury. If there was more than 1 level of injury, the level reported was the highest level of injury. The level of injury was divided in 2 categories: cervical and below cervical (ie, tetraplegia or paraplegia). Subjects were also asked to provide information concerning sociodemographic factors such as highest level of education and employment status. Education levels were classified as higher (defined as having an advanced, bachelor s, or associate s degree) or lower (pre high school, high school, trade school). Employment was either working (full time, part Table 1: Sociodemographic Data and Injury Characteristics (n 217) Mean Age (yr) Age at injury (yr) Time since injury (yr) time, student, self-employed) or not working (unemployed, retired, home maker) (table 1). Self-Reported Presence of Sadness, Excessive Fatigue, and Anxiety Subjects were asked to check whether they presently or within the last 3 months had experienced sadness, excessive fatigue, or anxiety. Frequency of Pain Interference The frequency of interference induced by pain was assessed in 5 areas: sleep, exercise, household chores, work, and other daily activities. The wording of the questions is shown below. Sleep. Two questions were asked: (1) How often is it difficult to go to sleep because of pain?; and (2) How often do you wake up because of pain? Six alternatives were given: (1) every night; (2) 3 to 6 times per week; (3) 1 to 2 times per week; (4) 1 to 3 times per month; (5) less than once per month; and (6) never. In the regression analysis, the following 2 categories were used: (1) high frequency interference (every night; 3 6 times/wk); and (2) low frequency interference (1 2 times/wk; 1 3 times/mo; 1 mo; never). This was to provide computable data for a logistical regression analysis. Exercise, household chores, and work. The following question was asked: How often does pain interfere with your exercise habits/your doing household chores/your work? Five alternatives were given in the questionnaire: (1) I do not normally exercise/take part in household chores/work; (2) SD n % Men 163 (75.1) Women 54 (24.9) Injury level Cervical 118 (54.4) Below cervical 97 (44.7) Not reported 2 (0.9) Highest level of education Pre high school 10 (4.6) High school 56 (25.8) Associate s degree 65 (30.0) Bachelor s degree 46 (21.2) Trade school 16 (7.4) Advanced degree 20 (9.2) Not reported 4 (1.8) Employment status Full time 44 (20.3) Part time 17 (7.8) Unemployed 83 (38.3) Student 24 (11.1) Retired 26 (12.0) Home maker 12 (5.5) Self-employed 7 (3.2) Not reported 4 (1.8) Abbreviation: SD, standard deviation.

3 CHRONIC PAIN AFTER SCI: LIFE INTERFERENCE, Widerström-Noga 1573 never; (3) sometimes; (4) often; and (5) always. The categories were divided the same way as above: (1) high frequency interference (often, always); and (2) and low frequency interference (never, sometimes). When a person indicated that he/ she did not take part in these activities because of reasons other than pain, the data were recorded as missing. Other daily activities. The following question was asked: How often does pain interfere with other daily activities? Four alternatives were given in the questionnaire: (1) never; (2) sometimes; (3) often; and (4) always (fig 2). The categories were divided the same way as above: (1) high frequency interference (often, always); and (2) and low frequency interference (never, sometimes). Location, Quality, and Intensity of Pain These data, representing the clinical characteristics of the chronic pain experienced by the participants, were described in more detail previously. 6 Location of pain (number of pain areas). Subjects were asked to mark on a pain drawing the areas corresponding to the chronic pain they were presently experiencing. The body areas were divided into 8 principal areas: (1) head; (2) neck and shoulders; (3) hands and arms; (4) frontal torso and genitals; (5) back; (6) buttocks; (7) thighs; and (8) legs and feet. These pain drawings have been presented previously in detail. 6 Only the number of areas marked by each individual was used in the logistical regression analyses to minimize the number of variables in the analysis. Quality of pain (number of descriptors). Subjects were asked to circle words from a list of 24 that best described the chronic pain they were presently experiencing. These data have been presented previously in detail. 6 Only the number of descriptors used by each individual was entered in the logistical regression analyses to minimize the number of variables in the analysis. Intensity of pain. Pain intensity was assessed using 2 separate NRSs, which ranged from 0 (no pain) to 10 (most intense pain imaginable). The subjects were asked to circle the numbers corresponding to their pain when most intense and to pain when least intense. Although the NRS only consists of 11 different levels, this has been previously reported to be adequate by individuals suffering from chronic pain. 31 These data have been presented previously in detail. 6 In the present study, the average pain intensity, ie, the mean of the most intense and least intense pain, was used in the analyses. Statistical Methods The statistical method used to predict frequency of interference was logistical regression analysis (stepwise procedure). 32,33 The logistical regression analysis is a type of multiple regression analysis based on maximum likelihood and is used when the dependent variable is categoric. 32 The automatic forward stepwise procedure starts with no variables in the model, and at each step, the most significant variable is entered. At each step the procedure examines the variables included for entry and removal until all variables in the model fulfil the criteria for retention. The odds of an event happening is the ratio of the probability of it happening to the probability of it not happening. This is presented in the odds ratio (OR) value. If the OR value is above zero for a particular variable, it means that the relationship is positive, ie, a high frequency of sleep interference due to pain is related to high pain intensity. If the OR value is below zero, the relationship is negative, ie, a high frequency of work interference due to pain is related to lower level of education. The OR value is often complemented with a 95% confidence interval (CI), indicating the range of numeric values in which we can be confident that 95% of the population value being estimated will be found. To define factors significantly predicting extent of interference, we used high versus low frequency interference (according to the definitions described above) for each of the areas of interference as the dependent variable. The independent variables were: (1) gender; (2) employment status; (3) level of injury; (4) age at injury; (5) time since injury; (6) level of education; (7) extent of pain (number of areas marked in the pain drawing); (8) number of pain descriptors; (9) average pain intensity (NRS); (10) excessive fatigue; (11) sadness; and (12) anxiety. RESULTS Sociodemographic Data and Characteristics of Injury Participants of this study had been injured for years and were on average years old when injured (table 1). Most subjects were men (75.1%) and cervical injures most common (54.4%). About 30% had advanced degrees or a bachelor s degree, and only 42.4% were employed full time or part time, students, or self-employed. Self-Reported Presence of Sadness, Excessive Fatigue, and Anxiety Subjects were asked to check whether they had experienced sadness, anxiety, or extreme fatigue during the last 3 months before the study. Sadness was most common; it was reported by 36% of our sample, followed by excessive fatigue (27.2%) and anxiety (26.3%). Frequency of Pain Interference Sleep. The frequency of sleep interference caused by pain is shown in figure 1. Interference with falling asleep as well as staying asleep is described below. Pain interfering with falling asleep. Eighty-two individuals (38.3%) experienced frequent interference, ie, 3 nights to every night per week. Forty-two (19.6%) persons experienced interference with falling asleep 1 to 2 times per week. Ninety Fig 1. Frequency of pain interfering with falling asleep and staying asleep. Frequency of pain interference is divided into 6 categories. The number of individuals reporting within each category is shown in percentages. (A) n 214, (B) n 215.

4 1574 CHRONIC PAIN AFTER SCI: LIFE INTERFERENCE, Widerström-Noga Fig 2. Frequency of pain interfering with (A) exercise (n 175), (B) household chores (n 135), (C) work (n 116), and (D) other activities (n 213). Frequency of pain interference is divided into 4 categories. The number of individuals reporting within each category is shown in percentages. individuals (42.1%) reported pain to interfere with falling asleep less than once per week to never. Three subjects chose not to report this area of interference (fig 1A). Pain interfering with staying asleep. Eighty-six individuals (40%) experienced interference 3 nights to every night per week. Thirty-four (15.8%) experienced interference with staying asleep 1 to 2 times per week. Ninety-five individuals (44.2%) reported pain to interfere with staying asleep less than once per week to never. Two subjects chose not to report this area of interference (fig 1B). Other Activities The frequency of interference caused by pain is shown in figure 2. The figure shows interference with exercise, household chores, work, and other daily activities. Exercise. Thirty-nine individuals reported that they did not normally exercise and 3 did not report. Of the remaining 175 persons, 61 (34.9%) reported that pain often to always interfered with their exercise. One hundred fourteen (65.1%) reported pain interfered sometimes to never with their exercise (fig 2A). Household chores. Seventy-five individuals reported that they did not normally do household chores and 7 did not report. Of the remaining 135 persons, 52 (38.5%) reported that pain often to always interfered with doing household chores whereas 83 (61.5%) reported pain to interfere sometimes to never (fig 2B). Work. Ninety-seven individuals reported that they did not work for reasons other than pain and 4 did not report. Of the remaining 116 persons, 39 (33.6%) reported that pain often to always interfered with their work. Seventy-seven (66.4%) persons reported pain to interfere sometimes to never with their work (fig 2C). Other daily activities. Of the 213 subjects, 79 (37.1%) reported that pain often to always interfered with other daily activities. One hundred thirty-four (62.9%) persons reported pain to interfere sometimes to never with other daily activities (fig 2D). Frequency of interference with areas of activity. Fortynine (22.5%) individuals reported they never experienced pain interference often to always with any of the listed activities. Pain interference was experienced often to always in 1 of the 6 areas by 19%, in 2 areas by 22.7%, in 3 areas by 12%, in 4 areas by 6.9%, in 5 areas by 8.8%, and in all 6 areas by 7.9%. For each of the 6 areas of activity, between 34% and 40% of the subjects reported experiencing frequent pain interference. Regression Analyses Predicting High Versus Low Frequency Interference Falling asleep. Two variables were significantly predictive of whether a person would have high or low frequency interference with falling asleep. Persons more likely to experience frequent interference used more descriptors when describing their pain (p.001) and reported a high average pain intensity (p.01) (table 2). Staying asleep. Four variables significantly predicted whether a person would have high or low frequency interference with staying asleep. Persons more likely to experience frequent interference were men (p.05), were older at time of injury (p.05), experienced anxiety the last 3 months (p.01), and had a high average pain intensity (p.001) (table 2). Exercise. Only 1 variable significantly predicted whether a person would have high or low frequency interference with doing exercise. Persons more likely to experience frequent interference with exercise were those who reported a high average pain intensity (p.001) (table 3). Chores. Four variables significantly predicted whether a person would have high or low frequency interference with performing household chores. Persons more likely to experience high interference were men (p.05), those who reported excessive fatigue (p.05), described their pain using multiple descriptors (p.05), and had high average pain intensity (p.01) (table 3). Work. Four variables significantly predicted whether a person would have high or low frequency interference with work. Persons more likely to experience frequent interference were older when injured (p.001), had a low level of education (p.05), reported anxiety (p.01), and described their pain using multiple descriptors (p.05) (table 3). Although average pain intensity was a predictor of frequent interference in all other areas of activity, it was not significantly associated with frequent interference with work.

5 CHRONIC PAIN AFTER SCI: LIFE INTERFERENCE, Widerström-Noga 1575 Table 2: Interference With Sleep OR 95% CI p Chi-Square (covariates) Falling Asleep (n 198) (2 df ) p.0001 Age at injury Anxiety Average pain intensity No. of descriptors Staying Asleep (n 197) (4 df ) p Age at injury Anxiety Average pain intensity No. of descriptors Other various daily activities. Three variables significantly predicted whether a person would have high or low frequency interference with performing or participating in various daily activities. Persons more likely to experience high interference were men (p.05), those who reported multiple pain locations (p.01), and had a high average pain intensity (p.001) (table 3). DISCUSSION Chronic pain after SCI imposes limitations in many important areas of an individual s life. The interference with daily activities may result in decreased independence, ultimately leading to depression and inactivity. 34 Our study showed that interference with common activities like sleep, household chores, exercise, work, and other daily activities was frequently caused by chronic pain. Pain interfered often to always with 1 or several of these different areas in 77.3% of subjects. Interference with going to sleep or staying asleep because of pain was reported often to always by 38.3% and 40%, respectively. The variables that most significantly predicted the extent of interference with falling asleep were pain described using multiple descriptors and high average pain intensity. Similarly, the frequency of interference with staying asleep was significantly predicted by a combination of factors including intense pain, but also by variables such as male gender, higher age at time of injury, and psychologic factors such as anxiety. Age at time of injury is an important factor for overall adjustment to injury and general well-being. 35 The association between frequent sleep disturbance and age at injury in combination with anxiety, male gender, and pain intensity observed in our study indicate that intense pain in men who were injured at a higher age and who felt anxious is likely to cause frequent Table 3: Interference With Chores and Other Daily Activities Activity/Variable OR 95% Cl p Chi-Square (covariates) Exercise (n 162) (1 df ) p.0001 Fatigue No. of descriptors Average pain intensity Chores (n 123) (4 df ) p.0001 Fatigue No. of descriptors Average pain intensity Work (n 106) (4 df ) p.001 No. of pain areas No. of descriptors Age at injury Education Anxiety Average pain intensity Other activities (n 196) (3 df ) p No. of pain areas No. of descriptors Age at injury Education Anxiety Average pain intensity

6 1576 CHRONIC PAIN AFTER SCI: LIFE INTERFERENCE, Widerström-Noga sleep interruption. Similarly, in a study by Edwards et al 36 comprising 215 persons with heterogenous chronic pain, significant relationships between high severity of pain, anxiety, and pain interference were obtained only in men. Daily household chores were performed by 62.2% of the present sample, and pain was commonly reported to interfere with these tasks. Frequent interference was more likely to be experienced by men feeling excessive fatigue and having intense widespread pain because the use of multiple descriptors is likely to be associated with several different types of pain. 37 Exercise is another important area of activity, particularly after SCI when exercise has profound beneficial effects on a variety of health issues. 36,38,39 In our study, 34.9% reported that pain frequently interfered with exercise and appeared to be dependent primarily on pain perceived as intense rather than any of the other assessed variables. Thus, demographic factors, such as gender, age at injury, level of injury, psychologic factors, or psychosocial factors such as employment or level of education seemed of less importance for interference with exercise induced by pain. Work opportunities for people with SCI are more limited than for able-bodied persons, and employment is often seen as an indicator of a successful rehabilitation or adaptation to injury. 40,41 Chronic pain thus imposes additional difficulty when interfering with a person s ability to work because a low level of engagement in work has been shown to be related to lower QOL after SCI. 42 Of the 53.5% (n 116) who worked or studied in our study, 33.6% reported frequent pain interference with work. The percentage of unemployment at 1 year after SCI, as reported from the national database, was 52.1%, but the likelihood for employment increases with years after injury. 43 Frequent pain interference with work was more likely when individuals suffered from several different types of pain, were older at the time of injury, had an education level at or below high school, and experienced anxiety. This relationship can be compared with the previously observed relationship between high education and a greater likelihood for employment after SCI. 43 Our study found that average pain intensity was not a predictor of frequent work interference, which concurs with data from a study by Rintala et al. 29 Interference with daily activities and sleep is directly related to a person s well-being and thus QOL. We found that frequent interference with daily activities, reported by 37.1%, was significantly predicted by male gender, widespread pain, and high pain intensity. Activities performed on a daily basis include a wide variety of tasks and a person with chronic widespread pain is likely to experience more interference due to pain than a person with less widespread pain. 23 Frequent interference with daily activities may result in lack of control in dealing with chronic pain. Low levels of internal control may result in negative affect and dependence on others. Similar relationships between disability, low levels of internal control, and depression have been observed in studies including heterogenous chronic pain samples. 44 Judging from the results of our study, chronic pain continues to be a problem in the SCI population, causing additional difficulty for people already dealing with the various consequences of SCI. We found that high pain intensity was among the most significant factors related to frequent pain interference. This indicates that even a decrease in intensity of pain may have significant beneficial effects on overall well-being. We showed in a previous study that pain was perceived as being very difficult to deal with by a large proportion of individuals with SCI 25 and that this rating correlated strongly with intensity of pain. 6 The relationship between pain intensity and interference has been established in heterogenous pain populations 24 using the MPI. The role of male gender in predicting frequent interference caused by chronic pain with chores and other daily activities may possibly be because of differences in gender roles, with women possibly being more active in domestic chores. 45 Another possibility for the association with male gender and frequent interference is the difference reported in use of coping strategies, with women engaging to a greater extent than men in problem solving, using social support, and positive self-statements. 46 Thus, women may not allow pain to interfere as frequently in activities judged as part of their daily routine. In contrast, in areas like exercise and work no gender differences were observed. The absence of a relationship between level of injury and extent of interference is not unexpected because level of injury is usually not associated with level of subjective well-being. 29,34 However, high life satisfaction after SCI has been shown to display positive relationships with factors like education, income, employment, and social and recreational activities, whereas medical complications are inversely correlated. 47 Furthermore, age and adjustment to life after injury are positively related. 21,22 Being injured at an older age is associated with a lower level of well-being, poorer health, and a less active life style. 35 Although the different types of pain after SCI are caused by different mechanisms, 9,11 the maintenance of the painful condition depends on a variety of factors, some of them unrelated to pathophysiology. 28 It is necessary to understand and treat pain after SCI optimally, not only to focus on the causative factors but also to define the factors responsible for sustaining these pain conditions. CONCLUSION We have presented interference frequencies in 5 common areas, ie, sleep, exercise, household chores, work, and other daily activities, and attempted to define factors influencing the extent of interference pain imposes on these areas. Using multivariate statistics and thus defining combinations of factors predicting frequency of interference induced by SCI pain in the different areas of activity is critical because such relationships may not be readily apparent in univariate relationships. Because of the close connection between lower QOL after SCI and chronic pain, it is important to define aspects of these pain conditions to increase our understanding and ultimately to improve pain management and well-being for individuals with SCI. Acknowledgments: The authors thank Barth Green, MD, for general support and encouragement, Jared Green for data entry, James Broton, PhD, for designing the database, and Robert Duncan, PhD, for statistical advice. References 1. 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