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Structured information during the ICU stay to reduce anxiety: study protocol of a multicenter randomized controlled trial
© Fleischer et al; licensee BioMed Central Ltd. 2009
Received: 22 May 2009
Accepted: 14 September 2009
Published: 14 September 2009
ICU stay is often associated with negative experiences for the individual patient. Many patients are disabled and their communication is restricted during the ICU stay. Specific information on procedures, sensations and coping behavior are thought to reduce anxiety on the ICU. Until now information programs to reduce anxiety were mainly delivered preoperatively, completely neglecting informational needs of non-elective ICU patients.
The trial is designed as a prospective multicenter randomized controlled trial in the cities of Marburg, Halle and Stuttgart. Elective and non-elective ICU patients will be included. The trial includes an intervention and a control group on the ICU. The control group receives a trivial conversation without any ICU-specific information. The intervention group receives an information program with specific procedural, sensory and coping information about their ICU stay. Both conversations take place in the ICU and are planned to take about 10 minutes.
In contrast to former trials on information programs on the ICU-stay our intervention will take place in the ICU itself. This approach will ensure to compensate for memory effects due to anesthesia or preoperative stress. Further the results will be applicable to non-elective ICU-patients.
ICU stay for ventilated patients is associated with severe impairments and limitations of communication. Additionally, these patients are restricted regarding their activities and ability to participate. These restrictions are mainly due to structural and procedural necessities including therapeutic interventions. The patients are not only handicapped by their vocal restraints, but also by the unknown and unfamiliar situation in the ICU. Nonetheless, ventilated patients are able to actively communicate with their environment. Research shows that nurse-initiated nurse-patient communication is lacking in this phase of ventilation . If the nurses are interacting with the patient, the interaction is nonverbal and rather instrumental. With the patient getting more and more awake and able to respond, nurse-initiated communication increases .
Research into the communication behavior in intensive care units, not limited to ventilated patients, showed interactions and interaction conditions which had predominantly been perceived as problematic by patients [2–10]. ICU patients often experience these ineffective communication situations with feelings of anxiety, insecurity but also irritation. Particularly ventilated patients evaluate the communication during their ventilation as non-successful [3, 10, 11]. In this context, Alasad and Ahmad  found that communication with patients in ICUs is not continuous and strongly depends on the patient's condition. Intensive care nurses found work with awakened patients being able to communicate verbally as more exhausting, as compared to still unconscious patients. Here, communication was also not perceived as a possibility of informing and supporting the patient.
Similarly, Scheer  observed that the nursing staff in ICUs often feels overstrained and tends to withdraw from the patients' bedsides. Similar results were obtained by means of a video analysis which investigated the communication and interaction between ventilated patients and nursing staff in an ICU . This analysis focused on the patient's remaining possibilities of expression in the different treatment stages taking into account also aspects of initiative and responsive communication. As a result, lessons in specific communicative abilities for nursing staff in intensive care were proposed, as the staff had acted upon "normal" social communication patterns and had not sufficiently observed the particularities of the setting .
The high priority of successful communication with communicatively challenged patients, however, is also appreciated by the ICU nursing staff [11, 13]. In their qualitative study, Usher and Monkley  investigated the point of view of nursing staff on successful and effective communication in an ICU and were able to identify three main issues: "perception of caregivers", "presence" and "strain relief" - whereas the category "strain relief" is mainly made up of the situational information of the patient. This perception of successful conscious communication is opposed by the notion that communication is always something random and natural . Against the background and as a consequence of Liedtkes  results, a concrete and conscious communication intervention for the information of the ICU patient and with "strain relief" in mind would be necessary.
Only one randomized controlled study (RCT), could be found which deals with a information intervention in the ICU . For their research approach in cardiac surgery patients, Hwang et al.  chose a recorded tape information from the attending physician which was played to the patients after recovering from anesthesia. The contents of the 6 to 8 minute recordings included the course as well as the result of the operation and further treatment plans. The intervention group showed significantly superior values regarding anxiety, tension, depression and pain perception compared to the control group. The results of this study suggest the effectiveness of postoperative situational information intervention for a positive influence on psychological parameters. Therefore a nursing care information program comparable to and modeled on the brief medical information is to be tested in the planned trial with ICU patients.
The aim of the project is to evaluate the effectiveness of a structured information program for patients who are staying in an ICU and thus having an associated induced communication restriction. In line with notions of control theory and nursing theory, the communication of knowledge is regarded as a means to enhance patients' capacity to understand, classify and predict the many unfamiliar events related to the ICU stay [15–17]. In this way it is not only possible to achieve an improvement of the difficult communication situation, but a cognitive reframing of the anxiety provoking ICU-stay. This can result in an increased perceived locus of control and eventually in stress reduction.
The key study hypothesis is:
An information program in the initial stage of treatment in an ICU alleviates unpleasant experiences related to the ICU stay and thus contributes to an improvement of the patient's situation and of particular aspects of quality of life, i.e. in relieving anxiety.
Besides this main point there are also other, secondary hypotheses particularly assuming effects on individual quality of life 3 months after discharge.
The trial is designed as a multicenter trial with concealed random allocation and central data monitoring (coordinating center: Marburg; 3 study centers). The multicenter design was chosen to increase the generalizability of the study results. Within the framework of the Medical Research Council [18, 19] this corresponds to the main phase of the evaluation of complex interventions, because the trial is designed as an extension to a previous study of our working group [20, 21]. Study methods include the randomization on the patient level and the delivery of the intervention by members of the study group. Potential contaminations of the intervention and control group will be avoided by restricting recruitment to one patient per room on the ICU at the same time.
Inclusion and exclusion criteria
Cardiac surgery, general surgery and internist patients with scheduled and unscheduled ICU stay (including High Dependency Units) will be eligible for inclusion in this trial. Exclusion criteria, besides a refusal to take part in the trial, are: no opportunity to fill in the mailed follow-up questionnaire, impaired cognitive ability and/or ability of judgment, language problems, accommodation in one room with another study patient, or age under 18 years.
Recruitment of participants
All patients who are to be admitted electively to the ICU will be informed about the possibility of taking part in the trial before the transfer to the ICU or before the surgery and their informed consent will be obtained. This applies to the major part of cardiac surgery patients as well as to a minor part of the general surgery patients. All other patients, particularly those of internal medicine and trauma surgery, will be informed about the trial in the early stages of their intensive care stay and will be asked to take part. In case custodians are appointed, they will be informed about the possibility of taking part in the trial and will be asked for a participation of their charge in the trial.
The group allocation is done via concealed randomization (concealed allocation) on the patient level, stratified by center and unit into 2 study groups:
▪ structured information at the beginning of the ICU stay (Intervention group) or
▪ sham comparator, unspecific conversation (Control group).
The randomization list will be computer-generated before the start of the trial (Institute of Medical Epidemiology, Biostatistics, and Informatics, Halle/Saale). Concealed randomization will be realized by using opaque, consecutively numbered envelopes after the inclusion of the study patients.
Patients and field researchers are not blinded to group allocation. A blinded statistician not involved in the trial process will be responsible for the final analysis.
The experimental intervention is planned as a conversation with ICU-specific information on the ICU during the first stage of the ICU stay. The control intervention is planned as a non-specific conversation of the same duration and at the same point of time as the experimental intervention.
During the information intervention a standardized and an individualized part are combined: a guided conversation corresponding to the individual choice in the dialogue. Thereby patients will be given sensory and procedural information (e.g. sources of noise in the ICU, chronology) as well as coping information (e.g.: What can you do if you are feeling strained?).
People in the ICU
• Health care professionals (nurses and intensive care nurses)
• Attending physician
• Clothing incl. specifics such as masks, gloves etc.
• Change of shifts
• Ward rounds
Devices and monitoring
• Monitor incl. central monitoring
• Infusion and syringe pump (infusomat and perfusor)
• Bell system
• Room size
• Tubes, drainages, wounds, urinary catheters, fixation
• Tube, respiratory mask
• Waking phase
• Intravenous access
• Dimming of the light
• Hospital stay duration
• Transfer to IMC
• Differences between IMC and ICU
• Nod, shake of the head
• Radiologic examinations
• Personal hygiene/oral hygiene
• Analgesics and soporifics (pain relievers and sleeping pills)
• Visiting hours
• Information before nursing-medical interventions
• Everything is done for me. That is sign that everything worked alright.
• I don't have to suffer any pain; if necessary I will receive additional medication. In the meantime I can relax and continue to breathe calmly.
• Only a little longer, then I have made it.
In the second part of the intervention the patients' individual information need on particular topics will be determined and further addressed. In addition, the patient will be given the opportunity to receive information on other topics.
Following preliminary studies, a length of about 10 minutes is scheduled for the intervention . A second intervention time will not explicitly be ruled out if the patient's information need could not be met with due to exhaustion, failing concentration or an interruption for other reasons.
The control patients will be offered a semi-structured, non-specific conversation with a member of the study group. This conversation with the patient will also take about 10 minutes. Priority will be given to topics such as mental state, experienced pain, sleeping habits and experienced anxiety.
Accompanying close relatives or friends can stay during the (control-) intervention if the participant agrees. As this can enhance the message, this will be documented in the case report form.
Before the experimental and control intervention the patients' level of consciousness and concentration will be recorded using the Richmond Agitation and Sedation Scale - RASS  and the Confusion Assessment Method for the ICU - CAM-ICU [25, 26]. The interventions will only be carried out at RASS values ≥ -3 and a negative test outcome of the four CAM-ICU criteria. In case of lower values the patient will be re-assessed at a later date (→ 24 h interval).
The study procedure will be carried out in each center by one of the study members who are not involved in the standard care in order to receive information on the efficacy. The comparability of the intervention taking place over the different centers is guaranteed by preliminary instructions of the study personnel for the implementation of the information program and the control intervention. Additionally, the coordinating center in Marburg will centrally monitor the whole trial.
In addition to the anxiety-related part of the CINT questionnaire a Visual Analogue Scale-Anxiety - VAS-A  supported by the Faces Anxiety Scale  is used. Evaluation of potential discrepancies between the retrospectively reported anxiety and the measured anxiety during the ICU-stay is of particular interest. The State and Trait Anxiety Inventory - STAI  as a further anxiety-related instrument will be recorded to compare anxiety after ICU-stay.
As a secondary variable, the patients' level of consciousness and concentration regarding a potential ICU-caused state of confusion will be surveyed with the Confusion Assessment Method for the ICU - CAM-ICU [25, 26]. The test procedure CAM-ICU, also appropriate for ventilated patients, comprehends cognitive as well as process-related components. These components are illustrated using the Richmond Agitation and Sedation Scale - RASS  and the four delirium main criteria "acute onset", "inattention", "disorganized thinking" and "altered level of consciousness".
For the analysis of additional effects the duration of stays (ICU, intermediate care, standard ward, hospital) and routine date on the course of inpatient treatment are recorded. Socio-demographic data like age, gender, marital status and routine treatment data like medical diagnoses will be documented thoroughly.
Uncertainty tolerance will be measured using the Uncertainty Tolerance Scale  and semi-structured questions on convalescence are asked to get a qualitative impression of the participants health state.
Expected results will contribute to an improved communication in the ICU and finally to an improved quality of life as a desired treatment goal [31–33]. Therefore we will measure quality of life using the Schedule for Evaluation of Individual Quality of Life - SEIQoL , and the SF-12 . For this reason we implemented a paper questionnaire version of the SEIQoL to test for acceptance of a self evaluation form for this kind of quality of life questionnaire.
day of recruitment (ICU)
▪ Socio-demographic data, routine treatment data2
▪ Acute ICU-Syndrome (CAM-ICU incl. RASS)2
▪ Anxiety (VAS-A)1
24 h after study intervention
▪ Acute ICU-Syndrome (CAM-ICU incl. RASS)2
▪ Anxiety (VAS-A)1
48 h after study intervention
▪ Acute ICU-Syndrome (CAM-ICU incl. RASS)2
▪ Anxiety (VAS-A)1
admission to standard ward
▪ Experience and emotional state in the ICU (CINT-FB)1
▪ Anxiety (VAS-A)1
▪ State-Anxiety (STAI-State)1
Estimated time for the questionnaire: 20 minutes
discharge from hospital
▪ In-patient history and postoperative complications2
▪ Length of stay and mode of discharge2
3 months after discharge (by mail)
▪ Individual quality of life (SEIQoL)1
▪ Quality of life (SF - 12)1
▪ Uncertainty (Uncertainty Tolerance Scale)1
▪ Convalescence (Semi-structured questions)1
▪ Trait-Anxiety (STAI-Trait)1
Estimated time for the questionnaire: 15 minutes
Sample size calculation
The sample size calculation is based on the anxiety-related part of the CINT questionnaire as particular aspects of the ICU-patients' quality of life . This sum score can be represented on a scale from 0 to 100 and has added up to a mean value of M = 28.0 and a standard deviation of SD = 17.0 in an earlier unpublished trial. The basis for the sample size calculation was formed by the following assumptions: α = 0.05; β = 0.20; Δ = 8.50 scale points. The determination of the expected difference Δ was done on the notion that in quality of life measures the differences within the scope of half a standard deviation are considered as minimally clinically relevant . Under these conditions at least 70 patients per group have to be included to ensure the effect statistically. In order to compensate potential drop-outs the sample size was determined n = 100 per group, corresponding to a total sample size of N = 200 (n = 100 vs. n = 100). In the centers Stuttgart and Halle respectively n = 30 patients per study group will be recruited, in Marburg n = 40 per study group.
Drop-outs will be documented thoroughly and included in the data analysis until the point of drop-out. Reasons for drop-out will be reported and analyzed.
Descriptive analyses will be carried out by standard methods, taking into consideration the scale of the respective variables. For analysis of the primary endpoint we will use an ANCOVA model with the primary endpoint as the response, and the center as a covariate, thus allowing for the stratified randomization procedure. The treatment effect will be reported as an adjusted difference in means with a corresponding 95% confidence interval. Secondary endpoints will be assessed with the respective adjusted models, using standard ANCOVA models for continuous outcomes and logistic regression for binary outcomes. In case of repeatedly measured outcomes, mixed models will be used for analyses to adjust for within-patient correlations . Secondary endpoints analyses are considered as merely exploratory. The analysis of all outcomes will obey to the intention-to-treat principle. Subgroup analyses will be conducted as interaction tests, also in a non-confirmatory fashion .
Protection of data privacy
We will create a pseudonym for all trial participants to collect and analyze the trial data. Key lists will be stored separately from the trial data and erased after final data analysis. Data will be analyzed in a way that no conclusions can be drawn to individual participants. Trial data is stored in lockable cabinets in lockable rooms.
The trial is part of the Nursing Research Network "Mitte-Süd". A report system is established within the network. Annual quality reports have to be prepared for the German Federal Ministry of Education and Research.
As the process of a multicenter trial demands high standards of quality to warrant comparable conditions and results among the centers all procedures were developed and documented in joint commissions.
We plan to publish the trial results in a peer-reviewed, international, Medline-listed journal, independent of study results. This mainly serves the purpose to avoid publication bias. Additionally, we are obliged by the Federal Ministry of Education and Research to report our results within 6 months after study termination. All trial results will be reported within context to this study protocol.
The study protocol is approved by the ethics committees of the universities in Marburg, Halle and Tübingen. If changes to the study procedures are necessary they will be proposed to the local ethics committees as amendments. All changes will be described and discussed in the publication of the trial's results.
In contrast to former trials on information programs on the ICU-stay our intervention will take place in the ICU. This approach will ensure to compensate for memory effects due to anesthesia or preoperative stress. Further the results will be applicable to non-elective ICU-patients. Thus our research will particularly contribute to the evidence on ICU-related information and communication.
The trial is funded by the German Federal Ministry of Education and Research (BMBF grant 01GT0601 and 01GT0603 as a project in the Nursing Research Network "Mitte-Süd").
We thank Prof. R. Moosdorf (University Hospital Giessen and Marburg, Location Marburg, Germany), and R. Thomiczny (Sana Herzchirurgische Klinik Stuttgart, Germany) for supporting the trial. Furthermore we want to thank S. Dyczek for her support with the English draft of the article.
- Liedtke A: Kann der beatmete Patient aktiv kommunizieren? Eine explorative Studie mittels Videoanalyse zur Interaktion zwischen beatmeten Patienten und Pflegepersonal auf einer Intensivstation. Hallesche Beiträge zu den Gesundheits- und Pflegewissenschaften. 2006, 5 (1):Google Scholar
- Alasad J, Ahmad M: Communication with critically ill patients. J Adv Nurs. 2005, 50 (4): 356-362. 10.1111/j.1365-2648.2005.03400.x.View ArticlePubMedGoogle Scholar
- Fowler SB: Impaired verbal communication during short-term oral intubation. Nurs Diagn. 1997, 8 (3): 93-98.View ArticlePubMedGoogle Scholar
- Hagland MR: Nurse-patient communication in intensive care: a low priority?. Intensive Crit Care Nurs. 1995, 11 (2): 111-115. 10.1016/S0964-3397(95)82022-1.View ArticlePubMedGoogle Scholar
- Llenore E, Ogle KR: Nurse-patient communication in the intensive care unit: a review of the literature. Aust Crit Care. 1999, 12 (4): 142-145. 10.1016/S1036-7314(99)70599-0.View ArticlePubMedGoogle Scholar
- Magnus VS, Turkington L: Communication interaction in ICU-Patient and staff experiences and perceptions. Intensive Crit Care Nurs. 2006, 22: 167-180. 10.1016/j.iccn.2005.09.009.View ArticlePubMedGoogle Scholar
- Rundell S: A study of nurse-patient interaction in a high dependency unit. Intensive Care Nurs. 1991, 7 (3): 171-178. 10.1016/0266-612X(91)90006-D.View ArticlePubMedGoogle Scholar
- Salyer J, Stuart BJ: Nurse-patient interaction in the intensive care unit. Heart & Lung. 1985, 14 (1): 20-24.Google Scholar
- Turnock C: Communicating with patients in ICU. Nurs Stand. 1991, 5 (15/16): 38-40.PubMedGoogle Scholar
- Wojnicki-Johansson G: Communication between nurse and patient during ventilator treatment: patient reports and RN evaluations. Intensive Crit Care Nurs. 2001, 17: 29-39. 10.1054/iccn.2000.1547.View ArticlePubMedGoogle Scholar
- Hafsteindottir TB: Patients' experiences of communication during the respirator treatment period. Intensive Crit Care Nurs. 1996, 12 (5): 261-271. 10.1016/S0964-3397(96)80693-8.View ArticlePubMedGoogle Scholar
- Scheer JW: Psychische Verarbeitung von Krankenhausaufenthalten. Medizinische Psychologie und Soziologie. Edited by: Wilker F, Bischoff C, Novak P. 1994, München: Urban und SchwarzenbergGoogle Scholar
- Usher K, Monkley D: Effective communication in an intensive care setting: Nurses' stories. Contemp Nurse. 2001, 10 (1-2): 91-101.View ArticlePubMedGoogle Scholar
- Hwang SL, Chang Y, Ko WJ, Lee MB: Stress-reducing effect of physician's tape-recorded support on cardiac surgical patients in the intensive care unit. J Formos Med Assoc. 1998, 97 (3): 191-196.PubMedGoogle Scholar
- Frey D, Jonas E: Die Theorie der kognizierten Kontrolle. Theorien der Sozialpsychologie Motivations-, Selbst- und Informationsverarbeitungstheorien. Edited by: Frey D, Irle M. 2002, Bern: Huber, 13-50.Google Scholar
- Shuldham C: A review of the impact of pre-operative education on recovery from surgery. Int J Nurs Stud. 1999, 36: 171-177. 10.1016/S0020-7489(99)00010-3.View ArticlePubMedGoogle Scholar
- Teasdale K: Information and anxiety: a critical reappraisal. J Adv Nurs. 1993, 18: 1125-1132. 10.1046/j.1365-2648.1993.18071125.x.View ArticlePubMedGoogle Scholar
- Medical Research Council (MRC): A framework for development and evaluation of RCTs for complex interventions to improve health. 2000, [http://www.mrc.ac.uk]Google Scholar
- Campbell NC, Murray E, Darbyshire J, Emery J, Farmer A, Griffiths F, Guthrie B, Lester H, Wilson P, Kinmonth AL: Designing and evaluating complex interventions to improve health care. BMJ. 2007, 334 (7591): 455-459. 10.1136/bmj.39108.379965.BE.View ArticlePubMedPubMed CentralGoogle Scholar
- Berg A, Fleischer S, Koller M, Neubert TR: Preoperative information for ICU patients to reduce anxiety during and after the ICU-stay: protocol of a randomized controlled trial [NCT00151554]. BMC Nurs. 2006, 5: 4-10.1186/1472-6955-5-4.View ArticlePubMedPubMed CentralGoogle Scholar
- Berg A, Fleischer S, Koller M, Moosdorf R, Neubert TR: Akzeptanz eines präoperativen Informationsprogramms zur Reduzierung von Angst während und nach Aufenthalt auf der Intensivstation. Optimierung und Evidenzbasierung pflegerischen Handelns: Ergebnisse und Herausforderungen der Pflegeforschung. Edited by: Schaeffer D, Behrens J, Görres S. 2008, Weinheim Juventa, 232-254.Google Scholar
- Neubert TR, Koller M, Bohrer T: Wie erleben Patienten den Aufenthalt auf einer chirurgischen Intensivstation? Eine prospektive Beobachtungsstudie aus Sicht der Pflege. Intensiv. 2004, 12: 1-10.Google Scholar
- Berg A, Fleischer S, Neubert TR, Koller M: Entwicklung und Pretest eines Programms mit spezifischen Informationen zum Intensivstationsaufenthalt. Hallesche Beiträge zu den Gesundheits- und Pflegewissenschaften. 2004, 3 (3): 18-20.Google Scholar
- Sessler CN, Gosnell MS, Grap MJ, Brophy GM, O'Neal PV, Keane KA, Tesoro EP, Elswick RK: The Richmond Agitation-Sedation Scale: validity and reliability in adult intensive care unit patients. Am J Respir Crit Care Med. 2002, 166 (10): 1338-1344. 10.1164/rccm.2107138.View ArticlePubMedGoogle Scholar
- Ely EW, Margolin R, Francis J, May L, Truman B, Dittus R, Speroff T, Gautam S, Bernard GR, Inouye SK: Evaluation of delirium in critically ill patients: Validation of the confusion assessment method for the intensive care unit (CAM-ICU). Crit Care Med. 2001, 29 (7): 1370-1379. 10.1097/00003246-200107000-00012.View ArticlePubMedGoogle Scholar
- Ely EW, Truman B, Ely EW: The confusion assessment method for the ICU (CAM-ICU). Training manual. 2002, Nashville, TN: MPH and Vanderbilt UniversityGoogle Scholar
- Chlan LL: Relationship between two anxiety instruments in patients receiving mechanical ventilatory support. J Adv Nurs. 2004, 48 (5): 493-499. 10.1111/j.1365-2648.2004.03231.x.View ArticlePubMedGoogle Scholar
- McKinley S, Coot K, Stein-Parbury J: Development and testing of a Faces Scale for the assessment of anxiety in critically ill patients. Journal of Advanced Nursing. 2003, 41 (1): 73-79. 10.1046/j.1365-2648.2003.02508.x.View ArticlePubMedGoogle Scholar
- Laux L, Glanzmann P, Schaffner P, Spielberger CD: Das State-Trait-Angstinventar: Theoretische Grundlagen und Handanweisung. Weinheim. 1981, Beltz TestgesellschaftGoogle Scholar
- Dalbert C: Die Ungewissheitstoleranzskala (UGTS). Diagnostische Verfahren in der Psychotherapie. Edited by: Brähler E, Schumacher J, Strauß B. 2003, Göttingen [u.a.]: Hogrefe, Verl. für Psychologie, 374-377.Google Scholar
- Behrens J: Die Verwechslung von Zielen und Mitteln und von interner und externer Evidence. Lebensqualitätsforschung in der Onkologie. Forum Deutsche Krebsgesellschaft. 2004, 4: 42-45.Google Scholar
- Koller M, Lorenz W: Quality of life: a deconstruction for clinicians. J R Soc Med. 2002, 95: 481-488. 10.1258/jrsm.95.10.481.View ArticlePubMedPubMed CentralGoogle Scholar
- Shuldham C: Pre-operative education - a review of the research design. Int J Nurs Stud. 1999, 36 (2): 179-187. 10.1016/S0020-7489(99)00011-5.View ArticlePubMedGoogle Scholar
- Hickey AM, Bury G, O'Boyle CA, Bradley F, O'Kelly FD, Shannon W: A new short-form individual quality-of-life measure (SEIQoL-DW); application in a cohort of individuals with HIV/AIDS. Br Med J. 1996, 313: 29-33.View ArticleGoogle Scholar
- Bullinger M, Kirchberger I: SF-36: Fragebogen zum Gesundheitszustand: Handanweisung. 1998, Göttingen [u.a.]: Hogrefe, Verl. für PsychologieGoogle Scholar
- Norman GR, Sloan JA, Wyrwich KW: Interpretation of changes in health-related quality of life: the remarkable universality of half a standard deviation. Med Care. 2003, 41 (5): 582-592. 10.1097/00005650-200305000-00004.PubMedGoogle Scholar
- Brown H, Prescott R: Applied mixed models in medicine. 2006, Chichester [u.a.]: Wiley, 2View ArticleGoogle Scholar
- Rothwell PM: Treating individuals 2. Subgroup analysis in randomised controlled trials: importance, indications, and interpretation. Lancet. 2005, 365 (9454): 176-186. 10.1016/S0140-6736(05)17709-5.View ArticlePubMedGoogle Scholar
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