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Clinical Investigations: ASTHMA |

Validation of a Standardized Version of the Asthma Quality of Life Questionnaire* FREE TO VIEW

Elizabeth F. Juniper, MSc; A. Sonia Buist, MD; Fred M. Cox, PhD; Penelope J. Ferrie, BA; Derek R. King, BMath
Author and Funding Information

*From the Department of Clinical Epidemiology and Biostatistics (Prof. Juniper, Ms. Ferrie, and Mr. King), McMaster University, Hamilton, Ontario, Canada; Oregon Health Sciences University (Dr. Buist), Portland, OR; and Glaxo Wellcome Inc (Dr. Cox), Research Triangle Park, NC. Supported by a grant from Glaxo Wellcome Inc.

Correspondence to: Elizabeth Juniper, MSc, Department of Clinical Epidemiology and Biostatistics, McMaster University Medical Centre, 1200 Main St West, Hamilton, Ontario, L8N 3Z5, Canada; e-mail: juniper@fhs.mcmaster.ca



Chest. 1999;115(5):1265-1270. doi:10.1378/chest.115.5.1265
Text Size: A A A
Published online

Background: In the original 32-item Asthma Quality of Life Questionnaire (AQLQ), five activity questions are selected by patients themselves. However, for long-term studies and large clinical trials, generic activities may be more appropriate.

Methods: For the standardized version of the AQLQ, the AQLQ(S), we formulated five generic activities (strenuous exercise, moderate exercise, work-related activities, social activities, and sleep) to replace the five patient-specific activities in the AQLQ. In a 9-week observational study, we compared the AQLQ with the AQLQ(S) and examined their measurement properties. Forty symptomatic adult asthma patients completed the AQLQ(S), the AQLQ, the Medical Outcomes Survey Short Form 36, the Asthma Control Questionnaire, and spirometry at baseline, 1, 5, and 9 weeks.

Results: Activity domain scores (mean ± SD) were lower with the AQLQ (5.7 ± 0.9) than with the AQLQ(S) (5.9 ± 0.8; p = 0.0003) and correlation between the two was moderate (r = 0.77). However, for overall scores, there was minimal difference (AQLQ, 5.4 ± 0.8; AQLQ(S), 5.5 ± 0.8; r = 0.99). Reliability (AQLQ intraclass correlation coefficient, 0.95; AQLQ(S) intraclass correlation coefficient, 0.96) and responsiveness (AQLQ, p < 0.0001; AQLQ(S), p < 0.0001) were similar for the two instruments. Construct validity (correlation with other measures of health status and clinical asthma) was also similar for the two instruments.

Conclusions: The AQLQ(S) has strong measurement properties and is valid for measuring health-related quality of life in asthma. The choice of instrument should depend on the task at hand.

Abbreviations: ACQ = asthma control questionnaire; AQLQ = Asthma Quality of Life Questionnaire; AQLQ(S) = standardized version of the AQLQ; ICC = intraclass correlation coefficient; PEF = peak expiratory flow; SF-36 = Medical Outcomes Survey Short Form 36

The Asthma Quality of Life Questionnaire (AQLQ) was developed to measure the problems that adults with asthma experience in their day-to-day lives.1Six independent studies have shown that it has strong measurement properties both as an evaluative and as a discriminative instrument.27 A feature of the AQLQ is that 5 of the 11 activity questions are selected by patients themselves. Many clinicians and investigators consider that this attribute enhances content validity, accommodates cultural differences, and improves relevance in clinical practice.89 However, for some studies in which this feature is less important, generic activities may be more appropriate. For instance, in long-term clinical trials and patient monitoring, patients’ activities may change over time, and in large clinical trials, the ease and convenience of standardized questions may outweigh the benefits of individualized questions. In addition, for cross-sectional surveys in which one is comparing burden of illness between individual patients or groups of patients, standardized questions are usually considered preferable.

We have developed a standardized version of the AQLQ, the AQLQ(S), in which five generic activities replace the five patient-specific activities of the AQLQ. In this validation study, we have compared scores generated by the AQLQ and AQLQ(S) and have examined the measurement properties that are important for the confident use of the AQLQ(S) in clinical trials, clinical practice, and cross-sectional surveys.

Formulation of the Five Generic Activities

Our aim was to formulate five generic activity questions that would cover the activities that are most frequently identified by asthma patients as being the most troublesome in their day-to-day lives. In addition, we wanted to ensure that the balance of the five questions (physical, social, occupational) reflects the frequency with which the activities are identified by patients. We reviewed the original item reduction data from the development of the AQLQ1 and a number of clinical trial databases in which the AQLQ had been an outcome. From these, we formulated five questions that we considered met the criteria identified above, and we grouped them according to quality-of-life function10 (Table 1 ). The wording of the five generic activity questions was pretested in 10 asthma patients to ensure ease and accuracy of understanding.

Validation Study

Subjects: Forty adults (18 to 65 years old) with current symptoms of asthma were enrolled from among patients who had participated in previous research studies and from among respondents to notices in the local media. Subjects were excluded if they had an illness, other than asthma, that might have an impact on health-related quality of life or if they had evidence of fixed airway obstruction (FEV1 after bronchodilator < 60% predicted). All participants understood the English version of the AQLQ and were able to make reliable measurements of peak expiratory flow (PEF). They all signed a consent form that had been approved by the McMaster University Faculty of Health Sciences Ethics Committee.

Study Design: In this 9-week observational study, patients were assessed at baseline and after 1, 5, and 9 weeks. At each clinic visit, spirometry was measured and the following questionnaires were completed: the self-administered version of the AQLQ plus the five new generic activity questions, the Asthma Control Questionnaire (ACQ),11and the Medical Outcomes Survey Short Form 36 (SF-36).12 For 1 week before each follow-up clinic visit, patients completed a daily diary.

Patients whose asthma was adequately controlled continued taking their established asthma medications throughout the study. Asthma was considered adequately controlled when none of the following were present: awoken at night by symptoms, persistent waking in the morning with symptoms, limitation of normal daily activities, sputum, inhaled short-acting β2-agonist > 2 puffs daily, or FEV1 after bronchodilator < 70% predicted. Patients whose asthma was not adequately controlled at week 1 or week 5 were advised to increase their dosage of medication in the manner recommended by their asthma consultant. For those without a treatment plan, we recommended an inhaled steroid, beclomethasone dipropionate, up to 800 μg daily.

Outcome Measures With AQLQ Plus Five Generic Activities: The AQLQ1 has 32 questions, the first 5 of which are the patient-specific activities. At the first clinic visit, each patient selects the five activities in which he or she has been most troubled by asthma during the previous 2 weeks. These activities are retained throughout the study. For this study, we used the self-administered version of the AQLQ, and the five generic activity questions were added to the end of the questionnaire. Patients respond to each question on a seven-point scale and recall their experiences during the previous 2 weeks. Results are expressed as four domain scores (symptoms, 12 questions; activities, 11 questions; emotional function, 5 questions; and environmental exposure, 4 questions) and as an overall score (32 questions).

Outcome Measures With ACQ: Ninety-two asthma clinicians from around the world identified the symptoms that are most important in determining clinical asthma control. The ACQ contains the five top-scoring symptoms, a question aboutβ 2-agonist use, and another about the FEV1 percent predicted (total of seven questions). The ACQ has been validated and shows strong measurement properties.11

Outcome Measures With Daily Diary: Each morning patients recorded the best prebronchodilator PEF of three blows and the amount of short-acting β2-agonist used in the previous 24 h.

Outcome Measures With SF-36: This 36-item generic health status questionnaire provides summary scores for physical and mental health. In patients with asthma, it has good internal consistency and cross-sectional validity.12

Statistical Analysis

First, we examined concordance between the AQLQ(S) and the AQLQ (criterion validity). Using data from the first clinic visit, we evaluated differences between AQLQ and AQLQ(S) with a paired t test and examined the closeness of association with a Pearson correlation coefficient.

Second, we compared the measurement properties of the two instruments. These included the following: (1) reliability, the ability of the instrument to give reproducible results when the clinical state is stable and to discriminate between patients with different levels of impairment; (2) responsiveness, the ability of the instrument to detect changes in clinical status even if that change is small; (3) cross-sectional validity, observed differences between patients truly reflect differences in asthma-specific quality of life (demonstrated by showing that correlations between both the AQLQ and the AQLQ(S) and other measures of asthma and generic health status are close to expected); and (4) longitudinal validity, observed changes in score with time truly represent changes in asthma-specific quality of life (demonstrated by showing that changes in AQLQ and AQLQ(S) scores correlate in a predictable manner with changes in other measures of health status).13 This analysis required that we categorize patients according to whether their asthma remained stable between consecutive visits (weeks 1 to 5 and weeks 5 to 9) or whether it changed.14 Categorization was performed using data from the ACQ in which a change in score≥ 0.5 on the seven-point scale can be considered clinically important.11 Patients whose conditions either improved or deteriorated by a score ≥ 0.5 were placed in the changed group. All other patients were placed in the stable group.

Reliability of the instruments was determined from patients in the stable group. If a patient was in stable condition between both weeks 1 and 5 and between weeks 5 and 9, a single observation was selected blindly using a random number generator. Reliability has been estimated as the within-subject SD and related to the total SD as an intraclass correlation coefficient (ICC). This statistic not only provides evidence of the instrument’s ability to give reproducible results but also its ability to discriminate between patients with different levels of impairment (1 = perfect reliability, 0 = no reliability).13

Responsiveness was determined in three ways. First, for patients in the changed group, we determined whether the instruments could detect these changes using a paired t test. Second, we determined whether the instruments could detect differences between the stable and changed groups using an unpaired t test. For both estimations, data from those whose conditions deteriorated and those whose conditions improved were combined by changing the sign of those whose conditions deteriorated after it had been shown that improvements and deteriorations in the patients in this study were symmetric. Third, we calculated the responsiveness index for each questionnaire. This index provides evidence of the degree of responsiveness of the instrument and is calculated by dividing change scores by the pooled SD of change.15The difference in responsiveness indices was tested using a paired t test. Some patients contributed two observations to a group. To ensure that this did not result in an overestimate of the precision of responsiveness, we inflated (multiplied) the variance to take into account within-subject correlations by the quantity 1 + (n − 1) ρ, where ρ is the ICC of the change scores and n = 2 (the number of observations per subject).16

All 40 patients completed the first three assessments; 1 patient failed to return for the final visit. Demographic data are shown in Table 2 .

Comparison of Instruments

The activities that patients selected most frequently for the patient-specific questions of the AQLQ are shown in Table 3 . As may be seen, all of these activities are well covered by the five generic activities of the AQLQ(S) (Table 1), and the balance of physical, social, and occupational activities in the generic questions is good.

The mean activity domain score was lower with the AQLQ (5.7 ± 0.9) than with the AQLQ(S) (5.9 ± 0.8), in which 7 = no impairment and 1 = maximum impairment. Although the mean difference between the two instruments was small, it still reached statistical significance (p = 0.0003). As a result, the overall quality-of-life score (mean ± SD) was slightly lower with the AQLQ (5.4 ± 0.8) than with the AQLQ(S) (5.5 ± 0.8), and this too reached significance (p = 0.0001). Correlation between the two activity domains was 0.77, but this made little difference to the overall correlation between the AQLQ and the AQLQ(S) (r = 0.99).

Measurement Properties

Reliability: Thirty-five patients remained in stable condition between two consecutive clinic visits and thus contributed to the reliability analysis. The within-subject SDs and ICCs for overall scores and individual domains are shown in Table 4 . In summary, the reliability of the AQLQ and the AQLQ(S) was very similar both for overall score and for the activity limitation domain.

Responsiveness: Twenty-six patients contributed 35 observations to the changed group; 35 patients contributed 44 observations to the stable group. Table 5 shows that responsiveness was strong in both instruments. They were both able to detect the within-subject changes in the changed group and to differentiate between the stable and changed groups. The responsiveness indices for the two instruments were very similar (AQLQ, 1.35; AQLQ(S), 1.34), and the difference was not significant (p = 0.35).

Construct Validity: Both cross-sectional and longitudinal correlations are sufficiently similar for the AQLQ and the AQLQ(S) (Tables 6 and 7 ) for us to have confidence that the AQLQ(S) is measuring the same construct as the AQLQ.

The results of this study provide evidence that the AQLQ(S) is a valid instrument for measuring health-related quality of life in adults with asthma. We evaluated validity in two ways: first, by determining the level of concordance (agreement) between the AQLQ(S) and the AQLQ (criterion validity); and, second, by comparing the measurement properties of the two instruments (construct validity).

The concordance analysis showed that the mean score for the activity domain of the AQLQ(S) was significantly higher (ie, less limitation) than for the activity domain of the AQLQ and that the correlation between the two activity domains was only 0.77. This lack of concordance is not surprising because patients select the activities that are most troublesome for the AQLQ and these activities may not all be captured by the generic activities in the AQLQ(S). In addition, one or more of the generic activities may be totally irrelevant to an individual patient (eg, a patient who never does any strenuous exercise). Although it is important to be aware of these differences between the two instruments, the mean difference for the overall score between the AQLQ and the AQLQ(S) was 0.1 and for the activity domain it was 0.2, both of which are considerably less than the minimal important difference of 0.52 and probably not of clinical importance. The second part of the analysis provided evidence that the measurement properties of the AQLQ(S) and the AQLQ (reliability, responsiveness, and both cross-sectional and longitudinal validity) are very similar and add further support for the validity of the AQLQ(S).

The correlations between both the AQLQ and the AQLQ(S) and other measures of clinical asthma status are very similar to those observed in a large number of other studies.27,11,16 They emphasize that within an individual patient, the problems and impairments that the patient experiences as a result of asthma cannot be inferred from the clinical measures of airway function; they have to be measured directly.

A limitation of this study is the relatively small sample size. However, other validation studies of the AQLQ would suggest that the sample size is quite adequate.24 In addition, patients may have been homogeneous. Although we tried to ensure that patients represented a wide range of asthma severity, age, and sex, the study was conducted in southern Ontario where the majority of patients are Caucasians. Testing of the AQLQ(S) in other settings will increase our confidence in the ability to generalize these results.

When selecting an instrument, it is important to review which properties are most important for the task at hand. There is no doubt that the patient-specific activities in the original AQLQ allow assessment of the problems that are important to the individual patient, and therefore this instrument can be considered to have better content validity than the AQLQ(S). This may be important if the instrument is being used in clinical practice to identify individual patient problems and the effect of interventions on these problems. It has also been suggested that patient-specific questions help to overcome cultural, climatic, and ethnic differences.89 In this study, we compared only the English versions of the AQLQ and the AQLQ(S), and further studies should be conducted to confirm the finding in other languages.

If the aim of a study is to compare burden of illness in a cross-sectional survey, the AQLQ(S) is probably better suited than the AQLQ. Although the discriminative properties of the AQLQ are strong and the patient-specific activities are only 5 of the 11 activity questions, individualized questions mean that the types and strenuousness of activities vary between patients.

The AQLQ(S) is also probably more appropriate than the AQLQ for long-term (> 1 year) clinical trials, cohort studies, and monitoring, when patients’ activities and priorities may change with time. For shorter periods, the decision becomes more difficult, and it is for each investigator to decide which priorities are more important. Responsiveness data from the two instruments are very similar, and, therefore, sample sizes for the AQLQ(S) (both parallel-group and crossover studies) will be very similar to those required for the AQLQ.2 Although it is more time-consuming to elicit patient-specific activities, the inclusion of these questions enables patients to feel that things that are important to them are being taken into consideration, and this may enhance compliance.

Most of these advantages and disadvantages are only speculative. Until further studies can provide evidence of the strengths and weaknesses of the two instruments in different clinical settings, the choice must rest on the individual clinician or investigator.

Table Graphic Jump Location
Table 1. Five Generic Activities
* 

If you are not employed or self-employed, these should be tasks you have to do most days.

Table Graphic Jump Location
Table 2. Patient Characteristics at Baseline*
* 

Data given as mean (SD). M = male; F = female.

 

Medications used in the previous 30 days.

Table Graphic Jump Location
Table 3. Patient-Specific Activities Chosen by Three or More Patients (AQLQ)
Table Graphic Jump Location
Table 4. Reliability of the AQLQ and the AQLQ(S) (n = 35)*
* 

The symptom, environmental stimuli, and emotional function domains are identical in the AQLQ and the AQLQ(S).

Table Graphic Jump Location
Table 5. Responsiveness Expressed as the Change in Score Between Consecutive Visits*
* 

The symptom, environmental stimuli, and emotional function domains are identical in the AQLQ and the AQLQ(S). Values given as mean (SD) unless otherwise indicated.

 

p < 0.001.

 

p < 0.01.

Table Graphic Jump Location
Table 6. Cross-Sectional Validity*
* 

The symptom, environmental stimuli, and emotional function domains are identical in the AQLQ and the AQLQ(S). Correlations are presented as positive whenever the association between the two variables is consistent with validity. Pearson correlation coefficients were used (n = 40).

Table Graphic Jump Location
Table 7. Longitudinal Validity*
* 

The symptom, environmental stimuli, and emotional function domains are identical in the AQLQ and the AQLQ(S). Correlations are presented as positive whenever the association between the two variables is consistent with validity. Pearson correlation coefficients were used (n = 40).

Juniper, EF, Guyatt, GH, Epstein, RS, et al (1992) Evaluation of impairment of health-related quality of life in asthma: development of a questionnaire for use in clinical trials.Thorax47,76-83. [PubMed] [CrossRef]
 
Juniper, EF, Guyatt, GH, Ferrie, PJ, et al Measuring quality of life in asthma.Am Rev Respir Dis1993;147,832-838. [PubMed]
 
Rutten-van Molken, MPMH, Clusters, F, Van Doorslaer, EKA, et al Comparison of performance of four instruments in evaluating the effects of salmeterol on asthma quality of life.Eur Respir J1995;8,888-898. [PubMed]
 
Rowe, BH, Oxman, AD Performance of an asthma quality of life questionnaire in an outpatient setting.Am Rev Respir Dis1993;148,675-681. [PubMed]
 
Leidy, NK, Coughlin, C Psychometric performance of the Asthma Quality of Life Questionnaire in a US sample.Qual Life Res1998;7,127-134. [PubMed]
 
Sanjuas, C, Alonso, J, Sanchis, J, et al The quality of life questionnaire with asthma patients: the Spanish version of the Asthma Quality of Life Questionnaire.Arch Bronconeumol1995;31,219-226. [PubMed]
 
Apter, AJ, Reisine, ST, Affleck, G, et al The influence of demographic and socioeconomic factors on health-related quality of life in asthma.J Allergy Clin Immunol1999;103,72-78. [PubMed]
 
McGee, H, O’Boyle, CH, Hickey, A, et al Assessing the quality of life of the individual: the SEIQoL with a healthy and a gastroenterology unit population.Psychol Med1991;21,749-759. [PubMed]
 
O’Boyle, CA, McGee, H, Hickey, A, et al Individual quality of life in patients undergoing hip replacement.Lancet1992;339,1088-1091. [PubMed]
 
Schipper, H, Clinch, J, Powell, V Definitions and conceptual issues. Spilker, B eds.Quality of life and pharmacoeconomics in clinical trials1996,11-23 Lippincott-Raven Publishers. Philadelphia, PA:
 
Juniper, EF, O’Byrne, PM, Guyatt, GH, et al Development and validation of the Asthma Control Questionnaire [abstract]. Am J Respir Crit Care Med. 1998;;157 ,.:A457
 
Bousquet, J, Knani, J, Dhivert, H, et al Quality of life in asthma: 1. Internal consistency and validity of the SF-36 questionnaire.Am J Respir Crit Care Med1994;149,371-375. [PubMed]
 
Guyatt, GH, Kirshner, B, Jaeschke, R Measuring health status: what are the necessary measurement properties?J Clin Epidemiol1992;45,1341-1345. [PubMed]
 
Juniper, EF, Guyatt, GH, Jaeschke, R How to develop and validate a new quality of life instrument. Spilker, B eds.Quality of life and pharmacoeconomics in clinical trials 2nd ed.1995,49-56 Raven Press. New York:
 
Guyatt, GH, Walter, S, Norman, G Measuring change over time: assessing the usefulness of evaluative instruments.J Chron Dis1987;40,171-178. [PubMed]
 
Juniper, EF, Guyatt, GH, Feeny, DH, et al Minimum skills required by children to complete health-related quality of life instruments: comparison of instruments for measuring asthma-specific quality of life.Eur Respir J1997;10,2285-2294. [PubMed]
 

Figures

Tables

Table Graphic Jump Location
Table 1. Five Generic Activities
* 

If you are not employed or self-employed, these should be tasks you have to do most days.

Table Graphic Jump Location
Table 2. Patient Characteristics at Baseline*
* 

Data given as mean (SD). M = male; F = female.

 

Medications used in the previous 30 days.

Table Graphic Jump Location
Table 3. Patient-Specific Activities Chosen by Three or More Patients (AQLQ)
Table Graphic Jump Location
Table 4. Reliability of the AQLQ and the AQLQ(S) (n = 35)*
* 

The symptom, environmental stimuli, and emotional function domains are identical in the AQLQ and the AQLQ(S).

Table Graphic Jump Location
Table 5. Responsiveness Expressed as the Change in Score Between Consecutive Visits*
* 

The symptom, environmental stimuli, and emotional function domains are identical in the AQLQ and the AQLQ(S). Values given as mean (SD) unless otherwise indicated.

 

p < 0.001.

 

p < 0.01.

Table Graphic Jump Location
Table 6. Cross-Sectional Validity*
* 

The symptom, environmental stimuli, and emotional function domains are identical in the AQLQ and the AQLQ(S). Correlations are presented as positive whenever the association between the two variables is consistent with validity. Pearson correlation coefficients were used (n = 40).

Table Graphic Jump Location
Table 7. Longitudinal Validity*
* 

The symptom, environmental stimuli, and emotional function domains are identical in the AQLQ and the AQLQ(S). Correlations are presented as positive whenever the association between the two variables is consistent with validity. Pearson correlation coefficients were used (n = 40).

References

Juniper, EF, Guyatt, GH, Epstein, RS, et al (1992) Evaluation of impairment of health-related quality of life in asthma: development of a questionnaire for use in clinical trials.Thorax47,76-83. [PubMed] [CrossRef]
 
Juniper, EF, Guyatt, GH, Ferrie, PJ, et al Measuring quality of life in asthma.Am Rev Respir Dis1993;147,832-838. [PubMed]
 
Rutten-van Molken, MPMH, Clusters, F, Van Doorslaer, EKA, et al Comparison of performance of four instruments in evaluating the effects of salmeterol on asthma quality of life.Eur Respir J1995;8,888-898. [PubMed]
 
Rowe, BH, Oxman, AD Performance of an asthma quality of life questionnaire in an outpatient setting.Am Rev Respir Dis1993;148,675-681. [PubMed]
 
Leidy, NK, Coughlin, C Psychometric performance of the Asthma Quality of Life Questionnaire in a US sample.Qual Life Res1998;7,127-134. [PubMed]
 
Sanjuas, C, Alonso, J, Sanchis, J, et al The quality of life questionnaire with asthma patients: the Spanish version of the Asthma Quality of Life Questionnaire.Arch Bronconeumol1995;31,219-226. [PubMed]
 
Apter, AJ, Reisine, ST, Affleck, G, et al The influence of demographic and socioeconomic factors on health-related quality of life in asthma.J Allergy Clin Immunol1999;103,72-78. [PubMed]
 
McGee, H, O’Boyle, CH, Hickey, A, et al Assessing the quality of life of the individual: the SEIQoL with a healthy and a gastroenterology unit population.Psychol Med1991;21,749-759. [PubMed]
 
O’Boyle, CA, McGee, H, Hickey, A, et al Individual quality of life in patients undergoing hip replacement.Lancet1992;339,1088-1091. [PubMed]
 
Schipper, H, Clinch, J, Powell, V Definitions and conceptual issues. Spilker, B eds.Quality of life and pharmacoeconomics in clinical trials1996,11-23 Lippincott-Raven Publishers. Philadelphia, PA:
 
Juniper, EF, O’Byrne, PM, Guyatt, GH, et al Development and validation of the Asthma Control Questionnaire [abstract]. Am J Respir Crit Care Med. 1998;;157 ,.:A457
 
Bousquet, J, Knani, J, Dhivert, H, et al Quality of life in asthma: 1. Internal consistency and validity of the SF-36 questionnaire.Am J Respir Crit Care Med1994;149,371-375. [PubMed]
 
Guyatt, GH, Kirshner, B, Jaeschke, R Measuring health status: what are the necessary measurement properties?J Clin Epidemiol1992;45,1341-1345. [PubMed]
 
Juniper, EF, Guyatt, GH, Jaeschke, R How to develop and validate a new quality of life instrument. Spilker, B eds.Quality of life and pharmacoeconomics in clinical trials 2nd ed.1995,49-56 Raven Press. New York:
 
Guyatt, GH, Walter, S, Norman, G Measuring change over time: assessing the usefulness of evaluative instruments.J Chron Dis1987;40,171-178. [PubMed]
 
Juniper, EF, Guyatt, GH, Feeny, DH, et al Minimum skills required by children to complete health-related quality of life instruments: comparison of instruments for measuring asthma-specific quality of life.Eur Respir J1997;10,2285-2294. [PubMed]
 
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