Each domain of health hasmany components: like symptoms, ability to function, and disability, that needto be measured. Because ofthis multidimensionality (Fig. 1), there is an almost infinite number of statesof health, all with differing qualities, and all quite independent oflongevity.

    Figure 1: Conceptual Scheme of the Domainsand Variables Involved in a Quality-of-Life Assessment.”x” axis represents subjective perceptionsof health, “y” axis objective health status, Coordinates Q (X, Y) the actual quality oflife, and “Z” measurement of the actual quality oflife associated with a specific component (e.g., positive affect) or domain (e.g.

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,the psychological domain).Measuring quality of life Translating the various domains andcomponents of health into a quantitative value that indicates the quality oflife is a complex procedure, drawing their tenants from the fields ofclinimetrics,20 psychometrics, and clinical decision theory.Usually most of the researchersmeasure each quality-of-life domain individually, by asking specific questionsrelated to its most important components. By asking a simple question, such as”Please rate your quality of life or overall health on a scale from 1 to 10,”although it may provide a useful global assessment, leaves “quality of life”and “overall health” ambiguously defined and the quantity being measured toovague to be interpreted more exactly. Relying purely on data indicatingobjective health status, such as physicians’ reports of symptoms, omits suchrelevant factors as a person’s threshold for the tolerance of discomfort.21Variation among quality-of-lifequestionnaires is often related to the degree to which they emphasize objectiveas compared with subjective dimensions, the extent to which various domains arecovered, and the format of the questions, rather than differences in the basicdefinition of quality of life.Constructingscales of measurementAs we know that many components ofquality of life cannot be observed directly, they are typicallyevaluated according to the classic principles of item measurement theory.

22According to this theory the truequality-of-life value, Q, cannot be measured directly, but it can be measuredindirectly by asking a series of questions known as “items,” each of whichmeasures the same true concept or construct. These set of questions are thenasked to the patient, and the answers are converted to numerical scores thatare then combined to yield “scale scores,” which may also be combined to yielddomain scores or other statistically computed summary scores.23If all the selected items have beenchosen properly, the resulting scale of measurement, Z, should differ from thecorresponding true value, Q, only by random error of measurement and shouldpossess several important properties.These properties are:1)   CoverageThe measurement of quality of life shouldcover each objective and subjective component (symptom, condition, or socialrole) that is important to members of the patient population and susceptible tobeing affected, positively or negatively, by interventions.2)   ReliabilityThe process of measurement must yieldvalues that are consistent or remain similar under constant conditions, even inan extended series of repeated assessments.3)   ValidityThe observed scales should be so valid; that is, they target andmeasure what they claim to measure.4)   ResponsivenessResponsiveness is a measure of theassociation between change in the observed score, Z, and the change in truevalue of the construct, Q. Since quality of life is not directlyobservable, a change in Q also cannot be measured directly.

So, responsivenessis often assessed by changing a criterion variable, C.5)   SensitivitySensitivity refers to the ability of themeasurement to reflect true changes or differences in Q (Quality of life).Selectingan Assessment InstrumentThe instruments and techniques usedto assess quality of life vary according to the identity of the respondent(that is, whether he or she is a clinician, patient, relative, or careprovider), the setting of the evaluation and the type of questionnaire used(short form, self-assessment instrument, interview, clinic-based survey,telephone query, or mail-back survey), and the general approach to theevaluation.Generic instruments are used ingeneral populations to assess a wide range of domains applicable to a varietyof health states, conditions, and diseases.24,25 Disease-specificinstruments focus on the domains most relevant to the disease or conditionunder study and on the characteristics of patients in whom the condition ismost prevalent.

26-28 Disease-specific instruments are mostappropriate for clinical trials in which specific therapeutic interventions arebeing evaluated.29-31The Oral Health Impact Profile (OHIP)questionnaire is one of the most commonly used instruments; it has been used invarious studies across different cultures and socio-demographic profiles. TheOHIP was developed in order to provide a comprehensive measurement of thedysfunction, discomfort, and disability associated with oral conditions asreported by the individual.32,33 OHIP analyzes the differentdimensions of functional patterns.

These dimensions are functionallimitation (e.g., difficulty chewing), pain (e.g.

, sensitivity of teeth),psychological discomfort (e.g., personal embarrassment), physical disability(e.g.

, changes in diet), psycho- logical disability (e.g., reducedconcentration), social disability (e.g., avoiding social contact), andincapacitation (e.

g., being unable to work productively).33,34In 1997, Slade described an abridged version of the OHIP, calledOHIP-14, which was derived from the original version, OHIP-49.32Among the 14 questions of OHIP-14, relate to the psychological and behavioralimpact and four address each of the remaining general dimensions. Therefore,OHIP-14 can be considered one of the best detectors of the psychosocial impactin a population.