Design
This is an observational, cross-sectional study for the validation of a measuring instrument, in which we have analyzed the construct validity, internal consistency, and concurrent validity of the MIDAS questionnaire. For test-retest reliability, a subsample of 29 subjects was evaluated twice with a time interval of 21 days.
Participants
The participants were young adult undergraduate and graduate students over 18 years. In order to be included, all participants were required to have been diagnosed with migraine. They were then examined by a physician (F.H.) to check if they fulfilled at that time the criteria described in the third edition of The International Classification of Headache Disorders [11].
We followed recommendations to include at least 10 subjects per item and to have at least 100 subjects for the purposes of the internal consistency and factor analysis [12]. A total of 230 subjects were contacted within the University of Jaén (Spain), of which 202, aged between 18 and 33, participated in this study between the months of March and June 2017. Finally, 153 participants met the inclusion criteria and completed all tasks. The selection process is shown in Figure 1.
The present study was approved by the Bioethics Committee of the University of Jaén (Reference number ABR 7/17) and was developed in accordance with the Helsinki Declaration, good clinical practices, and all applicable laws and regulations. All participants provided written informed consent prior to their inclusion in the study.
Measurements
Prior to completing the questionnaires, the participants reported their sociodemographic data including age, gender, weight, height, smoking habit, and degree of physical activity.
Two bilingual experts performed independent translations of the English version of the MIDAS questionnaire [6] into Spanish, following the guidelines recommended by the International Quality of Life Assessment project for cross-cultural translation [13]. Secondly, a consensus for a preliminary forward translation was reached between translators and researchers. Later, two bilingual experts performed a backward translation of the agreed Spanish version into English. The last English translation was compared with the original version of the MIDAS questionnaire in order to verify whether they had achieved semantic, linguistic, conceptual, and technical equivalence. Finally, 15 participants filled the Spanish version of the questionnaire to verify if the instructions, questions, and answering options were understandable.
The MIDAS questionnaire measures the degree of migraine-related disability experienced over the the last three months. It comprises seven items, the first five of which constitute the main scale and inquire about three distinct dimensions: workplace (two first items); domestic tasks (third and fourth items); and attendance to social, family, or leisure activities (fifth item). The sixth and seventh items refer respectively to the frequency and intensity of headaches over the last three months, and provide relevant data for clinicians to make informed decisions. The first six items must be answered with the number of days that headache conditioned any of the activities described in each question over the last three months. The seventh item is a Numeric Pain Scale, in which zero indicates absence of pain and ten indicates the maximum pain subjects believe they can withstand. According to the MIDAS-main scale score, which comprises the sum of the answers to items 1 to 5, subjects could be classified in 5 disability grades: Grade I (score 0-5): no disability or low disability; Grade II (score 6-10): mild disability; Grade III (score 10-20): moderate disability; and Grade IV (score >21): severe disability.
The 12-Item Short Form Health Survey (SF-12) was used in the present study to measure quality of life [14]. This is a self-administered questionnaire extracted from the SF-36 by means of multiple regression. The SF-12 consists of 12 items from which the physical and mental summary components (PCS-12 and MCS-12, respectively) yield a single score each. These two summary components showed high levels of internal consistency (Cronbach's alpha value of 0.85 for PCS-12 and 0.78 for MCS-12) [14].\
Statistical analysis
Data management and analysis were performed using the IBM SPSS Statistics package, version 23.0 (SPSS Inc., Chicago, IL) and the MedCalc statistical software, version 16.5.4 (MedCalc software, Inc., Mariakerbe). Data were expressed as means and standard deviations (SD) for continuous variables and as frequencies and percentages for categorical variables. The Kolmogorov-Smirnov test was used to test the normality of continuous variables. The level of statistical significance was set at P < 0.05.
A Principal Component Analysis (PCA) with varimax rotation was performed to measure the validity of the MIDAS construct. To test the feasibility of the factorial analysis we used Barlett's sphericity test. The suitability of the sample was analyzed using the Kaiser-Meyer-Olkin (KMO) test.
Internal consistency of the instrument was assessed through item analysis and the calculation of Cronbach's alpha. Values of Cronbach's alpha below 0.70 were considered weak, between 0.70 and 0.90 were considered good, and above 0.90 were interpreted as indicative of item redundancy [15].
Test-retest reliability was analyzed using the Intraclass Correlation Coeficient (ICC) as described by Shrout & Fleiss. Reliability was considered low for ICC values below 0.40, moderate for values between 0.40 and 0.75, high for ICC values between 0.75 and 0.90, and excellent for values higher than 0.90 [16].
Spearman’s correlation coefficient was used to analyze the tool’s concurrent validity with the SF-12 questionnaire. A correlation coefficient greater than 0.5 indicated a strong correlation, whereas values between 0.30 and 0.50 indicated a moderate correlation [17].