Subjective tinnitus is a frequent sensation of sound that cannot be attributed to an external sound source [1, 2]. Treatment of tinnitus is difficult and for most of the currently used treatment strategies the evidence of efficacy is low . Many interventions in reducing tinnitus-related distress are based on cognitive theories of behavior regulation and on psychological treatments [4, 5]. In recent years animal models and neuroimaging of tinnitus perception have contributed to substantial advances in the understanding of the pathophysiology of tinnitus [2, 6–8], which in turn has prompted the development of new treatment strategies [4, 9–11]. For assessing the efficacy of the various tinnitus treatment strategies, prospective placebo-controlled randomized trials have to be performed. An important aspect in the design of such clinical trials is the choice of the outcome measure. However in tinnitus research, the quantification of tinnitus severity can be challenging for several reasons. First, tinnitus is a purely subjective phenomenon and lacks any objectively identifiable variables or markers. Second, taking into account that tinnitus affects many different aspects of well-being (i.e., sleep, mood, concentration, energy), different patients may be more bothered by some symptoms and less by others .
In the evaluation of new treatments for tinnitus, several instruments are used to provide a quantification of tinnitus symptoms . The TQ is a widely used questionnaire for the quantification of tinnitus complaints. Developed by Hallam et al. , it has been translated and validated in German language  and is widely used in the German version. The TQ incorporates scales evaluating emotional and cognitive distress, intrusiveness, auditory perceptual difficulties, sleep disturbances, and associated somatic complaints. It was recommended, among other validated questionnaires, such as Tinnitus Handicap Inventory (THI) , Tinnitus Handicap Questionnaire (THQ) , and Tinnitus Reaction Questionnaire (TRQ) , in a consensus document of the Tinnitus Research Initiative (TRI) to be used as an outcome measurement in clinical trials . It has also been used, in isolation or in conjunction with other tinnitus questionnaires, for assessing the effect of various therapeutic interventions in many clinical studies on chronic tinnitus [9, 18–20].
When using health status questionnaires to ascertain whether a treatment for a given condition is effective or not, statistical significance of effects is usually reported. Statistically significant effects are those that are beyond a certain level of chance. However, noteworthy statistical significance of a treatment effect largely depends on the sample size and does not provide information of whether observed changes are clinically meaningful. In contrast, clinical relevance of a treatment effect refers to its impact upon the patient, to its implications for management of the patient, to its ability to meet standards of efficacy set by patients, clinicians, and researchers [21, 22]. The question of what is the clinical meaning of the reported score change usually remains open [21–23]. Thus, deriving clinical meaning from statistically significant results may be misleading. Results of clinical studies for tinnitus usually report changes in TQ scores as continuous variables for each group  (e.g., mean change or Effect Size (ES) for each group) and thus are difficult to interpret when translated to the level of clinical relevance and an individual response. Therefore, exact knowledge about which change of the TQ is clinically relevant is critical, both for the design and interpretation of a clinical trial as well as for rational decision-making in clinical management of tinnitus patients. In order to estimate meaningfulness of changes in clinical scores, the concept of the Minimal Clinically Important Difference has been developed [25, 26]. The MCID for a given questionnaire score can be defined as the value above which the change becomes clinically relevant.
However, no consensus exists on the methods that should be used in estimating the MCID . Techniques used in MCID evaluation are usually divided in two groups: distribution- and anchor-based. Distribution-based methods use statistical properties of a study's results, e.g., ES, Standard Error of Measurement (SEM) and other measures obtained from characteristics of study population [21–23, 27, 28]. In anchor-based methods changes in used measuring instrument (e.g., patient reported outcomes, PRO) are referenced to an anchor [21, 25, 26, 29], which should reflect the patient’s perspective . This is especially relevant for a purely subjective condition such as tinnitus .
Given that tinnitus is a purely subjective condition a patient-rated CGI seems more appropriate as a judgment for the change of tinnitus-related global impairment than a CGI of a rater, based on an interview. Accordingly, several recent clinical trials used a patient-rated CGI change as outcome criterium [32, 33]. Therefore, patient-rated CGI scales represent a valid example of a reference anchor [26, 34, 35]. Anchor based methods were recommended as primary methods for MCID estimation complemented by various distribution-based estimates (e.g., ES and SEM) as supportive information [25, 36, 37].
It has been implied in the available literature that the magnitude of a meaningful score change may be independent of the direction of change, i.e., MCID for improvement (MCID-I) is equivalent to the MCID for deterioration (MCID-D) [26, 38]. However, clinical experience and previous studies challenge this assumption [37, 39]. Thus MCID can be bidirectional and can differently reflect an improvement and deterioration. It has been observed in studies in tinnitus and chronic pain patients that a smaller change in measuring instrument scores is sufficient to feel deterioration than the change needed to feel improvement [31, 39, 40]. In contrast studies in cancer patients showed the opposite. Small improvements were considered relevant by patients whereas declines have to be large to be perceived as worsening [37, 41].
The aim of this study was to determine which change in the TQ score is considered as a meaningful clinical change with the main aim to identify the minimal score reduction which is perceived as improvement (MCID-I) [37, 39]. We defined MCID as the smallest change in the measurement instrument used that signifies a perceptible improvement or deterioration in tinnitus from the patient’s perspective. To estimate clinically relevant changes, we analyzed data from the TRI database  and from the RESET study . We compared changes in the TQ with patient’s subjective evaluation of treatment-related changes of tinnitus assessed with the Clinical Global Impression Change scale.