Updated: November 2016
It is widely believed, and seems likely, that regular, high-quality sleep is important for personal performance and well-being, as well as for public safety and other important outcomes. Unfortunately, many people are unable to fall asleep as quickly as desired and/or unable to stay asleep as long as desired — a condition known as insomnia. Reliable and scalable treatments for insomnia could bring substantial humanitarian benefit.
The Open Philanthropy Project hopes to survey a wide range of potential cause areas in the social sciences, only some of which will turn out to look promising enough to warrant deeper investigation and potential grantmaking. We chose to conduct a brief, surface-level investigation of the evidence for the effectiveness of standard behavioral treatments for insomnia because we thought it might turn out to be promising enough to warrant deeper investigation, and because it seemed to be a well-contained topic on which we could experiment with variations on our process for generating such reports (more on this below). We might or might not investigate non-behavioral treatments for insomnia later.
I (Luke Muehlhauser) had two goals for this project: to identify the most well-regarded behavioral treatments for insomnia, and then evaluate what the state of the evidence on those treatments’ effectiveness appears to be. I did not attempt to closely examine any studies I found — mostly, I only evaluated “surface features” such as what methods the study authors claim to have used.
My overall conclusion, described in more detail here, is that I don’t think we have strong evidence to suggest that standard behavioral treatments for insomnia are effective at ≥1mo after treatment.
To increase the speed with which we can survey the evidence concerning many different potential cause areas, we decided not to invest as much time on exposition and thoroughness as we have for some other investigations.
If you know of studies or reviews which seem like they should have been mentioned or cited in this report, or which are important but were published after the initial release of this report, please send them to [email protected] along with your comments, if any.
1 My process
For this report, I will describe my literature search process in less detail than I did for my carbs-obesity report. This time, we experimented with a different investigation process that we hoped would require less overhead on our part but still allow the report to be vetted for accuracy by Open Philanthropy Project staff and by external readers. To that end, the sections describing my tentative conclusions about treatments for insomnia are footnoted with vettable probability statements such as “To be more precise: I am X% confident that my spreadsheet of RCTs on this topic includes at least Y% of RCTs on this topic which have features A, B, and C.” We suspect these probability statements will be useful only for some readers. I explain our motivations for providing these probability statements in more detail in a footnote.1
Here is a brief account of my literature search process. First, I searched for general overview articles on treatments for insomnia,2 and quickly learned that the relevant literature is organized under the heading of “sleep medicine.” I used these general overview articles to familiarize myself with the standard concepts, treatments, and outcome measures used in the field.
Once I learned that standard insomnia treatments have been tested by many randomized controlled trials (RCTs),3 I decided to focus only on the evidence for treatment effectiveness from RCTs. I then searched for systematic reviews (SRs) of RCTs for insomnia treatments, and found that the literature could be divided into three major categories of treatments: psychological/behavioral treatments for insomnia (I’ll call them “BTIs”), pharmacological treatments for insomnia (“PTIs”), and alternative treatments for insomnia (“ATIs”) such as acupuncture. For now, I decided to investigate only BTIs.
To survey SR-included RCTs testing the effectiveness of commonly-tested BTIs, I did the following:
- I made a spreadsheet (here) of all the SRs of RCTs (plus other studies, in some cases) testing the effectiveness of any kind of insomnia treatment, published online before October 2015. I found ~70 such SRs, and I think this is a fairly complete list.4
- I identified the SRs on this list that focused entirely or mostly on BTIs (rather than PTIs or ATIs).
- Two GiveWell staff members5 identified all unique RCTs across these SRs,6 and identified which ones met certain criteria discussed below, for example which outcome measures were used at each study’s last follow-up assessment. I spot-checked their work.7 Our spreadsheet of SR-included RCTs is available here.
- I quickly reviewed the RCTs matching certain criteria, and wrote my tentative conclusions below.
An initial draft of this report was internally vetted in April and May of 2016 by Sarah Ward, which led to a few minor error corrections. We considered publishing the details of the vet and the edits it prompted, but in this case doing so would have been prohibitively time-expensive, and we decided not to do so.
2 How insomnia treatments are studied
First let me set the stage for my later substantive claims about BTIs by explaining some basic concepts related to BTIs, as explained by recent narrative reviews on the topic.8
A patient with insomnia can’t fall asleep as quickly as they’d like, and/or can’t stay asleep as long as they’d like. Insomnia with one or more obvious medical, psychiatric, or environmental causes (e.g. acute pain) is known as comorbid or secondary insomnia; otherwise the condition is known as primary insomnia. Common BTIs for primary or comorbid insomnia include:
- Sleep restriction: Instruction to avoid the bed as much as possible when not sleeping in it.9
- Stimulus control: Instructions which aim to strengthen the mental and physiological association between the bed and sleep, and to establish a regular sleeping schedule. E.g.: “Go to bed only when sleepy,” “Get out of bed when unable to sleep,” “No napping,” and “Arise at the same time every morning.”10
- Sleep hygiene: Education about health practices (e.g. diet, exercise, substance use) and environmental factors (e.g. noise, light, temperature) that may affect sleep success.11
- Relaxation training: Procedures aimed at reducing arousal, muscle tension, and thoughts that may interfere with sleep, e.g. meditation and progressive muscle relaxation. Most of these procedures require some initial training and practice.12
- Cognitive therapy: Psychotherapy aimed at treating anxiety about sleep problems and reframing false beliefs about insomnia.13
- Cognitive behavioral therapy for insomnia (CBT-I): A combination of several different treatments from the above list, perhaps most commonly of sleep restriction, stimulus control, and sleep hygiene.14
Hereafter, I’ll refer to these BTIs as “standard” BTIs.15
CBT-I appears to be the most commonly-discussed BTI in the research literature, and is plausibly the most common BTI in clinical practice. It can be delivered on an individual basis or in a group setting, via self-help (with or without phone support), and via computerized delivery (with or without phone support). It can also be delivered simultaneously with PTIs and ATIs.16
In RCTs testing the effectiveness of standard BTIs, night-time sleep outcomes are typically measured with one or more of the following measures:
- Polysomnography (PSG): PSG combines objective measures of brain activity, eye movement, muscle activity and perhaps also heart rhythm, respiration, blood oxygen saturation, and other measures. PSG is widely considered the “gold standard” measure of sleep, but it has several disadvantages. It is expensive, complicated to interpret, requires some adaptation by the patient (people aren’t used to sleeping with wires attached to them), and is usually (but not always) administered at a sleep lab rather than at home.17
- Actigraphy (ACT): An actigraph is a watch-like device that uses an accelerometer to record movement during the night. Supposedly (I haven’t checked), it correlates well with PSG on at least two key variables — total sleep time (TST) and sleep efficiency (SE: percentage of time in bed spent asleep) — in healthy subjects, but agreement rates are lower in patients with insomnia. Actigraphy is less expensive and more convenient than PSG, and can easily be used at home.18
- Sleep diary (SD): Subjects are asked to fill out a daily diary of sleep outcomes, usually including TST, SE, sleep onset latency (SOL: how long it took to fall asleep), wake-time after initial sleep onset (WASO), and perhaps other variables. Usually, subjects are asked to self-report these outcomes in the morning, for the previous night’s sleep. Supposedly (I haven’t checked), SD is known to be less accurate than PSG or ACT, but it seems to be the most common measure of sleep outcomes in RCTs of BTIs.19
- Questionnaires: A variety of standardized questionnaires are available to measure sleep outcomes, the most common of which is probably the Pittsburgh sleep quality index (PSQI). The PSQI includes 19 questions about sleep quality over the past month, and results in a total score for overall sleep quality as well as 7 component scores (e.g. sleep duration and sleep quality). I haven’t checked how valid and reliable this measure is.20
In the section below, I focus on measurements of TST and SE, because these two variables seem (to me) to capture the most relevant outcome information without requiring that I check the results for a cumbersomely long list of outcome variables, and because they are two of the most commonly measured outcome variables.
3 How effective are commonly tested BTIs?
To quickly assess the likely effectiveness of commonly tested BTIs, I looked only at SR-included RCTs testing the effectiveness of standard BTIs for adults (or mostly adults). Approximately 180 unique RCTs were included across all the SRs I found (published online before October 2015).21
3.1 Long-term effectiveness, measured objectively
First, I looked at RCTs that (1) measured the long-term (≥6mo) effectiveness of one or more BTIs, and that (2) used at least one objective measure of sleep (PSG or ACT) during the last follow-up measurement.22
These criteria yielded ~20 RCTs. Unfortunately, only 7 of these RCTs had a neutral control, retained it through a follow-up period of at least 6 months (thus allowing meaningful comparisons between active treatment and neutral control at that follow-up), and reported objectively-measured TST or SE at that follow-up.23 The results of these studies, focusing on objectively-measured TST and SE, are:
|STUDY [LAST FOLLOW-UP]||TREATMENT CONDITIONS AT LAST FOLLOW-UP||PARTICIPANTS (AT LAST FOLLOW-UP)||OBJECTIVELY MEASURED TST AND SE AT LAST FOLLOW-UP|
|Lichstein et al. (2001) [12mo]||Relaxation therapy vs. sleep compression vs. placebo desensitization||74 subjects from Memphis, 59 or older, chronic primary insomnia, no sleep apnea, no sleep medications, plus some other criteria||PSG: TST and SE were worse for relaxation therapy subjects than placebo subjects. Sleep compression subjects averaged ~40 more minutes of TST than placebo subjects, and ~7 percentage points greater sleep efficiency. Statistical significance of these differences not reported.24|
|Wu et al. (2006) [8mo]||CBT-I vs. placebo tablets25||36 subjects26 from an unspecified location (Beijing?), chronic primary insomnia, no sleep apnea, no sleep medication, plus some other criteria||PSG: CBT-I subjects averaged ~53 more minutes of TST than controls, and ~10 percentage points greater sleep efficiency. Statistical significance of these differences not reported.27|
|Berger et al. (2009) [12mo]||CBT-I vs. healthy eating instructions28||155 female subjects from the U.S. Midwest, breast cancer-related fatigue, receiving chemotherapy, no pre-cancer insomnia or sleep apnea, plus some other criteria||ACT: CBT-I patients averaged 16 more minutes of TST than controls, and 1 percentage point higher “sleep percent after onset.” Statistical significance of these differences not reported.29|
|Espie et al. (2008) [6mo]||CBT-I vs. treatment as usual (TAU)||106 subjects from Scotland, chronic insomnia, diagnosed with cancer, no sleep apnea, plus some other criteria||ACT: No effect of CBT-I over TAU for either TST or SE.30|
|Edinger et al. (2005) [6mo]||CBT-I vs. sleep hygiene vs. TAU||20 subjects from an unspecified location (near Durham, NC?), insomnia and fibromyalgia but not other comorbidities, no sleep apnea, plus some other criteria||ACT: No group differences for either TST or SE.31|
|McCurry et al. (2014) [18mo]||CBT for pain vs. CBT for pain and insomnia vs. education only||320 subjects, members of a health maintenance organization in Washington state (“Group Health”), aged 60 or older, had received care for osteoarthritis at Group Health in the past 3 years, with chronic pain and insomnia, no sleep apnea, plus some other criteria32||ACT: TST not measured. No differences between groups for SE.33|
|Lichstein et al. (2013) [12mo]||CBT vs. placebo biofeedback vs. withdrawal||61 subjects from (or near) Memphis, diagnosed with hypnotic-dependent insomnia, aged 50 or older, no sleep apnea, plus some other criteria||PSG: Probably no group differences for either TST or SE (but statistical significance not reported).34|
The results of these studies are inconsistent. Though three of these seven studies did not report the statistical significance of the comparisons that most interested me,35 I would guess (based on effect sizes) that the first two studies (as listed above) each found positive and statistically significant effects of at least one BTI on TST and SE (objectively measured, at last follow-up) that are both statistically significant and large enough to be “practically” significant, whereas the last five studies (as listed above) found no statistically or practically significant effects (objectively measured, at last follow-up).
Moreover, these seven trials were only moderately pragmatic in design.36 For example, subject eligibility was usually tightly restricted, resulting in a sample of subjects that is not especially representative of the population we’d like to treat for insomnia. All else equal, I consider pragmatic trials to provide stronger evidence of broad intervention effectiveness than explanatory trials do, for reasons described here.
3.2 Medium-term effectiveness, measured objectively
Perhaps it is too much to hope for that we could have good evidence that BTIs are effective ≥6mo after treatment. What if we look at standard BTIs’ effects on objectively measured TST and SE at the last follow-up occurring ≥1mo and ≤3mo after treatment?
These criteria yielded 16 RCTs. Unfortunately, only 3 of these RCTs had a neutral control, retained it through the designated follow-up period, and reported objectively-measured TST or SE at the designated follow-up period.37 The results of these studies, focusing on objectively measured TST and SE, are:
|STUDY [LAST FOLLOW-UP ≥1MO AND ≤3MO AFTER TREATMENT]||TREATMENT CONDITIONS AT DESIGNATED FOLLOW-UP||PARTICIPANTS (AT DESIGNATED FOLLOW-UP)||OBJECTIVELY MEASURED TST AND SE AT DESIGNATED FOLLOW-UP.|
|Wu et al. (2006) [3mo]||CBT-I vs. placebo tablets38||36 subjects39 from an unspecified location (Beijing?), chronic primary insomnia, no sleep apnea, no sleep medication, plus some other criteria||PSG: CBT-I subjects averaged ~71 more minutes of TST than controls, and ~17 percentage points greater sleep efficiency. Statistical significance of these differences not reported.40|
|Lovato et al. (2014) [3mo]||CBT-I vs. wait list control||99 subjects from near Adelaide, South Australia, chronic insomnia, no sleep apnea, plus some other criteria||ACT: CBT-I subjects averaged ~30 fewer minutes of TST than controls. No difference for sleep efficiency.41|
|Berger et al. (2009) [3mo]||CBT-I vs. healthy eating instructions||160 female subjects from the U.S. Midwest, breast cancer-related fatigue, receiving chemotherapy, no pre-cancer insomnia or sleep apnea, plus some other criteria||ACT: CBT-I patients averaged 10 more minutes of TST than controls, and 1 percentage point higher “sleep percent after onset.” Statistical significance of these differences not reported.42|
The results of these studies are inconsistent. The first study listed above reported a CBT-I advantage that is plausibly practically and statistically significant, whereas the other two studies did not. Moreover, as with the RCTs summarized in the previous section, these three trials were only moderately pragmatic in design.43
3.3 Immediate effectiveness, measured via self-report
Finally, what if we look at self-reported TST and SE, immediately after treatment? This is the kind of summary statistic typically reported in meta-analyses of RCTs on the topic. Here are the findings from the most recent (2015-2016) SRs of RCTs of standard BTIs I reviewed:
|SR||FOCUS OF THE SR||INCLUDED RCTS||BASIC RESULTS FOR SELF-REPORTED TST AND SE, AT POST-TREATMENT|
|Johnson et al. (2016)||CBT-I for cancer survivors||8||“CBT-I resulted in a 15.5% improvement in SE relative to control conditions.” TST not reported.|
|Geiger-Brown et al. (2015)||CBT-I for comorbid insomnia||23||Standardized mean difference for TST was .25 and for SE was .93.44|
|Koffel et al. (2015)||Group CBT-I||8||Mean effect size for TST was -.04 and for SE was .84.45|
|Ho et al. (2015)||Self-help CBT-I||20||Mean effect size for TST was .24 and for SE was .80.46|
|Zacharie et al. (2015)||Internet-delivered CBT-I||11||Hedges’ g for TST was .29 and for SE was .58.47|
|Trauer et al. (2015)||CBT-I, excluding studies focused on comorbid insomnia||20||“TST improved by 7.61… minutes, and SE improved by 9.91%.”|
In short, these SRs tend to report practically relevant average effects on SE but not so much for TST.
However, I don’t summarize more details from these SRs, or summarize details from any SR with an official publication date earlier than 2015, because I don’t weight their meta-analytic findings very heavily in my consideration of the evidence, for two reasons.
First, I don’t trust the accuracy of self-reported sleep diary measurements. In part, this is because some (but not all) narrative reviews on insomnia report that sleep diaries are considered a less accurate measure of sleep than PSG or ACT.48 And while I couldn’t find any SRs of studies comparing self-report and objective measures of sleep in adults, I did find two SRs of studies comparing self-report (or parent-report) of sleep and objective measures in children and adolescents, and both of those SRs reported low correspondence between self-report/parent-report and objective measures.49 Moreover, both a priori reasoning about self-report measures and empirical reviews of the accuracy of self-report measures (across multiple domains) lead me to be suspicious of self-reported measures of sleep.50 Finally, it’s my impression, from the dozens of studies I skim-read for this investigation, that objective and self-report measures of sleep often disagree, with the self-report measures typically showing more beneficial effects of treatment than objective measures show.51
Second, I’m interested in lasting effects of treatment, not immediate post-treatment effects.
Finally, a point that applies to studies using either self-report measures or objective measures or both: I expect few to no RCTs on this topic to be both high quality and highly pragmatic.52
Standard BTIs have only rarely been tested against a neutral control at ≥1mo follow-up using objective measures of TST or SE in RCTs, and these results are inconsistent, with most such studies showing no practically significant effect of treatment at the follow-ups I checked. Moreover, I would guess that standard BTIs have never been tested in this way in a high-quality, highly pragmatic RCT. Given this, and given that I have many reasons to be suspicious of self-report measures of sleep quality, I don’t think we have strong evidence to suggest that standard BTIs are effective at ≥1mo.
I would be quite surprised if a more thorough search for RCTs testing the effectiveness of standard BTIs challenged this tentative conclusion.53
If I were to substantially change my mind about this upon further investigation, my guess is that the most likely reasons for this change of mind would be:
- There turn out to be reasons to think self-reported sleep diary measurements of sleep are more accurate than I currently suspect they are, and a well-designed recent meta-analysis of RCTs relying on sleep diary measurements shows substantial positive effects of standard BTIs at ≥1mo (when sleep diary measures are used), in a variety of populations and contexts.
- There is at least one well-conducted, highly pragmatic RCT which shows that a standard BTI improves sleep outcomes at ≥1mo (using objective measures), but I didn’t find this RCT in my search. If I found one well-conducted pragmatic RCT of this nature, that could be more persuasive to me than meta-analyses of many small, weak, mostly explanatory RCTs, for reasons described here.
Despite my skepticism about the state of the evidence on the effectiveness of standard BTIs, I continue to suggest some standard BTIs (in particular sleep restriction and sleep hygiene) to insomnia sufferers who ask me for advice. I make this suggestion not based on scientific evidence, but based on my intuitive priors about which interventions seem to me like they might work, and the fact that these interventions are usually cheap to try.
In other words, my personal recommendation that insomnia sufferers at least try the sleep restriction and sleep hygiene treatments is given from the following perspective: “The effectiveness evidence in this area is weak. But sleep restriction and sleep hygiene seem intuitively to me like they might help at least some insomnia sufferers, and that’s not true of most possible insomnia treatments one could propose (e.g. various herbal treatments, about which I have no intuitions concerning effectiveness). If you’ve got insomnia, you might as well try sleep restriction and sleep hygiene and see whether they help you. But if I wanted to predict how much human welfare (via insomnia reduction) would accrue if someone spent several million dollars improving or scaling the delivery of standard BTIs, I would say I have no idea because the scientific evidence is too weak to allow me to make that kind of judgment, even as a guesstimate.”
4 What might I recommend funding in this area?
Obviously, I would want to investigate this topic more deeply before making any funding recommendations. But if I had to guess, on the basis of what I know now, which funding recommendations I’d make upon investigating further, I would guess I’d end up recommending something like the following.
Before anyone funds the first large, expensive, highly pragmatic RCT on this topic, I think we should make sure we’ve got an accurate and ecologically valid measure of sleep, and I’m worried that current actigraphs aren’t accurate enough, even if they’re more accurate than sleep diaries. So, I’d be curious to learn more about the feasibility of developing a night-time sleep measure that will strongly agree with PSG for approximately all populations and conditions. It seems to me like this might be feasible, plausibly via a combination method: e.g. perhaps a comfortable-to-wear headband or skullcap, plus an improved actigraph, and maybe also some little device that listens to one’s breathing throughout the night (or even something similar to this micro-CPAP device54 but only for measuring respiration). Basically: if the startup incubator X (formerly Google X) wanted to build a highly accurate measure of sleep that didn’t require attaching wires to people, what would they build?
If we had a highly accurate measure that subjects could use relatively cheaply at home — either because a new measure was developed or because actigraphy looks more accurate to me upon deeper investigation than it does now — then my next step would probably be to recommend a relatively small pre-registered RCT with ≥6mo follow-up, open data, blinding for everything that can be blinded, and so on — just to see if we could get some preliminary good news about person-delivered CBT-I vs. computerized CBT-I vs. placebo once we’re using an accurate and ecologically valid sleep measure and checking off basic methodological boxes like pre-registration. I’d also want to make sure more development effort goes into the computerized CBT-I intervention than is usually the case.
If one such RCT was promising, or perhaps only if a few such RCTs were promising, then I might be ready to recommend a large, well-designed, multi-site, highly pragmatic RCT with ≥6mo follow-up, testing the effectiveness of person-delivered CBT-I vs. computerized CBT-I vs. placebo.
I have very little sense of how much these things would cost. My guess is that if a better measure of sleep (of the sort I described) can be developed, it could be developed for $2M-$20M. I would guess that the “relatively small” RCTs I suggested might cost $1M-$5M each, whereas I would guess that a large, pragmatic RCT of the sort I described could cost $20M-$50M. But these numbers are just pulled from vague memories of conversations I’ve had with people about how much certain kinds of product development and RCT implementation cost, and my estimates could easily be off by a large factor, and maybe even an order of magnitude.
|Adamo et al. (2009)||Source|
|Airing micro-CPAP||Source (archive)|
|Barnow & Greenberg (2014)||Source (archive)|
|Barsevick et al. (2010)||Source (archive)|
|Bastien et al. (2012)||Source|
|Bauer & Blunden (2008)||Source (archive)|
|Belanger et al. (2007)||Source (archive)|
|Berger et al. (2009)||Source (archive)|
|Bhandari & Wagner (2006)||Source (archive)|
|Bound et al. (2001)||Source (archive)|
|Bryant et al. (2014)||Source (archive)|
|Cochrane Collaboration Risk of Bias Tool||Source (archive)|
|Currie et al. (2000)||Source (archive)|
|Donaldson & Grant-Vallone (2002)||Source (archive)|
|Edinger et al. (2005)||Source (archive)|
|Espie et al. (2008)||Source (archive)|
|Fayers & Machin (2016)||Source (archive)|
|Fernandez-Ballesteros & Botella (2007)||Source|
|Fiorentino et al. (2010)||Source (archive)|
|Geiger-Brown et al. (2015)||Source (archive)|
|Geretsegger et al. (2012)||Source (archive)|
|Google Scholar||Source (archive)|
|Gorber et al. (2007)||Source (archive)|
|Gorber et al. (2009)||Source (archive)|
|Groves et al. (2009)||Source (archive)|
|Hauri (1981)||Source (archive)|
|Ho et al. (2015)||Source (archive)|
|Hoch et al. (2001)||Source (archive)|
|Hodge et al. (2012)||Source (archive)|
|Johnson et al. (2016)||Source (archive)|
|Jungquist et al. (2012)||Source (archive)|
|Koffel et al. (2015)||Source (archive)|
|Kowalski et al. (2012)||Source (archive)|
|Kryger et al. (2016), 6th Edition||Source|
|Kryger et al. (2016), 5th Edition||Source (archive)|
|Kuncel et al. (2005)||Source (archive)|
|Lichstein et al. (2001)||Source (archive)|
|Lichstein et al. (2012)||Source|
|Lichstein et al. (2013)||Source (archive)|
|Loudon et al. (2015)||Source (archive)|
|Lovato et al. (2014)||Source (archive)|
|Luke Muehlhauser, Insomnia treatment SRs||Source|
|McCurry et al. (2007)||Source (archive)|
|McCurry et al. (2014)||Source (archive)|
|Meyer et al. (2009)||Source (archive)|
|Miller et al. (2015)||Source|
|Morin & Benca (2012)||Source (archive)|
|Morin et al. (1999)||Source (archive)|
|Open Philanthropy Project, RCTs included in SRs on behavioral treatments of insomnia||Source|
|Payne et al. (2008)||Source (archive)|
|Perlis et al. (2010)||Source (archive)|
|Price et al. (2011)||Source (archive)|
|Prince et al. (2008)||Source (archive)|
|Schwarz et al. (2008)||Source|
|Smith (2011)||Source (archive)|
|Smith et al. (2002)||Source (archive)|
|Stalans (2012)||Source (archive)|
|Stone et al. (1999)||Source (archive)|
|Stone et al. (2007)||Source (archive)|
|Streiner & Norman (2008)||Source (archive)|
|Suziedelyte & Johar (2013)||Source (archive)|
|Taylor et al. (2014)||Source (archive)|
|Thomas & Frankenberg (2002)||Source|
|Trauer et al. (2015)||Source (archive)|
|Vitiello et al. (2013)||Source (archive)|
|Wikipedia, X||Source (archive)|
|Wood & McCall (2013)||Source (archive)|
|Wu et al. (2006)||Source (archive)|
|Zacharie et al. (2015)||Source (archive)|